protect $@ whenever a block using $@ is non-trivial

As noted in the Try::Tiny man page, eval/$@ can be quite awkward in
corner cases, because $@ has the same properties and problems as C's
errno. While writing a regression test for definetemplate
in which it couldn't find an appropriate template, I received

    <span class="error">Error: failed to process template
    <span class="createlink">deftmpl</span> </span>

instead of the intended

    <span class="error">Error: failed to process template
    <span class="createlink">deftmpl</span> template deftmpl not
    found</span>

which turned out to be because the "catch"-analogous block called
gettext before it used $@, and gettext can call define_gettext,
which uses eval.

This commit alters all current "catch"-like blocks that use $@, except
those that just do trivial things with $@ (string interpolation, string
concatenation) and call a function (die, error, print, etc.)

IkiWiki/CGI.pm

@@ -351,7 +351,8 @@ sub cgi_getsession ($) {
 			{ FileName => "$config{wikistatedir}/sessions.db" })
 	};
 	if (! $session || $@) {
-		error($@." ".CGI::Session->errstr());
+		my $error = $@;
+		error($error." ".CGI::Session->errstr());
 	}
 	
 	umask($oldmask);

IkiWiki/Plugin/aggregate.pm

@@ -553,7 +553,9 @@ sub aggregate (@) {
 			};
 		}
 		if ($@) {
-			$feed->{message}=gettext("feed crashed XML::Feed!")." ($@)";
+			# gettext can clobber $@
+			my $error = $@;
+			$feed->{message}=gettext("feed crashed XML::Feed!")." ($error)";
 			$feed->{error}=1;
 			debug($feed->{message});
 			next;
@@ -675,7 +677,9 @@ sub write_page ($$$$$) {
 		$template=template($feed->{template}, blind_cache => 1);
 	};
 	if ($@) {
-		print STDERR gettext("failed to process template:")." $@";
+		# gettext can clobber $@
+		my $error = $@;
+		print STDERR gettext("failed to process template:")." $error";
 		return;
 	}
 	$template->param(title => $params{title})

IkiWiki/Plugin/attachment.pm

@@ -229,8 +229,10 @@ sub attachment_store {
 		check_canattach($session, $final_filename, $tempfile);
 	};
 	if ($@) {
-		json_response($q, $form, $dest."/".$filename, $@);
-		error $@;
+		# save error in case called functions clobber $@
+		my $error = $@;
+		json_response($q, $form, $dest."/".$filename, $error);
+		error $error;
 	}
 
 	# Move the attachment into holding directory.

IkiWiki/Plugin/editpage.pm

@@ -400,10 +400,12 @@ sub cgi_editpage ($$) {
 		eval { writefile($file, $config{srcdir}, $content) };
 		$config{cgi}=1;
 		if ($@) {
+			# save $@ in case a called function clobbers it
+			my $error = $@;
 			$form->field(name => "rcsinfo", value => rcs_prepedit($file),
 				force => 1);
 			my $mtemplate=template("editfailedsave.tmpl");
-			$mtemplate->param(error_message => $@);
+			$mtemplate->param(error_message => $error);
 			$form->tmpl_param("message", $mtemplate->output);
 			$form->field("editcontent", value => $content, force => 1);
 			$form->tmpl_param("page_select", 0);

IkiWiki/Plugin/edittemplate.pm

@@ -130,9 +130,11 @@ sub filltemplate ($$) {
 		$template=template("/".$template_page);
 	};
 	if ($@) {
+		# gettext can clobber $@
+		my $error = $@;
 		# Indicate that the earlier preprocessor directive set 
 		# up a template that doesn't work.
-		return "[[!edittemplate ".gettext("failed to process template:")." $@]]";
+		return "[[!edittemplate ".gettext("failed to process template:")." $error]]";
 	}
 
 	$template->param(name => $page);

IkiWiki/Plugin/inline.pm

@@ -391,7 +391,9 @@ sub preprocess_inline (@) {
 					blind_cache => 1);
 			};
 			if ($@) {
-				error sprintf(gettext("failed to process template %s"), $params{template}.".tmpl").": $@";
+				# gettext can clobber $@
+				my $error = $@;
+				error sprintf(gettext("failed to process template %s"), $params{template}.".tmpl").": $error";
 			}
 		}
 		my $needcontent=$raw || (!($archive && $quick) && $template->query(name => 'content'));

IkiWiki/Plugin/mdwn.pm

@@ -92,8 +92,9 @@ sub htmlize (@) {
 					$markdown_sub=\&Markdown::Markdown;
 				}
 				else {
+					my $error = $@;
 					do "/usr/bin/markdown" ||
-						error(sprintf(gettext("failed to load Markdown.pm perl module (%s) or /usr/bin/markdown (%s)"), $@, $!));
+						error(sprintf(gettext("failed to load Markdown.pm perl module (%s) or /usr/bin/markdown (%s)"), $error, $!));
 					$markdown_sub=\&Markdown::Markdown;
 				}
 			}

IkiWiki/Plugin/template.pm

@@ -41,9 +41,11 @@ sub preprocess (@) {
 			blind_cache => 1);
 	};
 	if ($@) {
+		# gettext can clobber $@
+		my $error = $@;
 		error sprintf(gettext("failed to process template %s"),
 			htmllink($params{page}, $params{destpage},
-				"/templates/$params{id}"))." $@";
+				"/templates/$params{id}"))." $error";
 	}
 
 	$params{basename}=IkiWiki::basename($params{page});