API formatting of errors and exceptions

Currently any API request that results in an error response results in an error message which is one of:
- a stack trace
- a generic http error response which hides the original message

Error responses should exhibit the following:
- an appropriate HTTP response code set
- a message which is clear and helpful to the user
- where appropriate, some machine-parsable reference to where in the template the source of the error is (parameter name for validation error, resource name for a resource error, etc)

python-heatclient and horizon should display any message and make use of any reference to template elements.