[#1011] Fix a 500 to a 404

Catch Genshi TemplateNotFound errors when rendering the package read
page. This can happen if an IDatasetForm plugin returns a custom package
read template, e.g. 'read.html', but the user has requested the dataset
in RDF format and the plugin does not provide a corresponding 'read.rdf'
template. (CKAN will take the path to read.html, replace the filename
extension with rdf, try to render this non-existing template file, and
crash.)

Replace this crash with a proper 404.

Add to the IDatasetForm docs, explaining how to provide RDF templates
for datasets.

Fixes #1011.
[#1011] Refactor an exception raise and catch

Raise and catch a CKAN exception, instead of a Genshi one.
Merge branch 'master' of github.com:okfn/ckan into 1011-template-not-found
[#1183] Show very simple indicator for job status on resource page
[#1183] Fix url
[#938] Fix datapusher test
[#1183] Add snippet for datapusher status indicator
[#1183] Clean up template code
[#1183] Only show for users who can edit resource
[#1183] Don't use get_action because it will be deprecated in #1193
[#938] Use datapusher_enabled action instead of checking whether it is enabled ourselves
[#1183] Add file that I forgot
[#1183] Add file that I forgot
[#1196] Move datapusher code into separate extension
Merge branch 'master' into 1183-datapusher-status
Merge branch '1196-separate-datapusher' into 1200-datastore-read-only
[#938] Validate datapusher_submit with a schema
[#1200] Imports and comments clean up
[#1200] Do not allow writing if url_type != datastore unless force=true is passed
[#938] Prepend config option with ckan.