Error handling: Model-level validations, proper error responses.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	moto/ec2/responses/instances.py
Error handling: Model-level validations, proper error responses. (added coverage for new instance status call)
Error handling: Model-level validations, proper error responses. (removed deprecated error template)
Error handling: Model-level validations, proper error responses. (backport assert_raises as context manager for Python 2.6)
Error handling: Model-level validations, proper error responses. (fixed typo)
Merge pull request #167 from DreadPirateShawn/ErrorHandling

Error handling: Model-level validations, proper error responses.