Fix error message consistency.

examples/error_not_found.html

@@ -0,0 +1 @@
+{{foo}}

examples/error_not_found.js

@@ -0,0 +1 @@
+var error_not_found = {bar: 2};

examples/error_not_found.txt

@@ -0,0 +1 @@
+ERROR: 'foo' not found in context

mustache.js

@@ -136,7 +136,7 @@ var Mustache = function() {
       if(context[name] !== undefined) {
         return context[name];
       }
-      throw("Can't find " + name + " in " + context);
+      throw({message: "'" + name + "' not found in context"});
     },
 
     // Utility methods

test/mustache_spec.rb

@@ -34,10 +34,10 @@ describe "mustache" do
       try {
         print(Mustache.to_html("{{#list}}{{x}}{{/list}}", {list: [{}]}));
       } catch(e) {
-        print('ERROR: ' + e);
+        print('ERROR: ' + e.message);
       }
     JS
-    run_js(js).should == "ERROR: Can't find x in [object Object]\n"
+    run_js(js).should == "ERROR: 'x' not found in context\n"
   end
   
   non_partials.each do |testname|