More info for undefined template error

Closes https://github.com/phoenixframework/phoenix/issues/266

This PR does the ff:
1. Adds test case for undefined template error
2. Include Module name of View in undefined template error

Wasn't sure where to put the test case. Was thinking either in `template_test` or in `view_test`. I put it in `view_test` since adding it in `template_test` led to a little more set-up(requiring files and aliases). Let me know if it was unnecessary to add the test case.