show appropriate error message when revert wont apply

lib/gollum/frontend/app.rb

@@ -97,6 +97,7 @@ module Precious
         @versions = [sha1, sha2]
         diffs     = wiki.repo.diff(@versions.first, @versions.last, @page.path)
         @diff     = diffs.first
+        @message  = "The patch does not apply."
         mustache :compare
       end
     end

lib/gollum/frontend/templates/compare.mustache

@@ -7,13 +7,20 @@
     <ul class="actions">
       <li class="minibutton"><a href="/history/{{escaped_name}}"
          class="action-page-history">Back to Page History</a></li>
+    {{#show_revert}}
       <li class="minibutton">
         <input type="submit" id="gollum-editor-submit" value="Revert" title="Save current changes">
       </li>
+    {{/show_revert}}
     </ul></div>
   </form>
   {{>searchbar}}
 </div>
+
+{{#message}}
+<p>{{message}}</p>
+{{/message}}
+
 <div id="compare-content">
   <div class="data highlight">
     <table cellpadding="0" cellspacing="0">

lib/gollum/frontend/views/compare.rb

@@ -1,7 +1,7 @@
 module Precious
   module Views
     class Compare < Layout
-      attr_reader :page, :diff, :versions
+      attr_reader :page, :diff, :versions, :message
 
       def title
         "Comparison of #{@page.title}"
@@ -30,6 +30,10 @@ module Precious
         lines
       end
 
+      def show_revert
+        !@message
+      end
+
       # private
 
       def line_class(line)