Consider invalid keywords a template validation error

This commit changes the exception type raised when an invalid
keyword is found in the resources and outputs sections of hot
templates from a KeyError to a StackValidationFailed. The
latter is better suited to be sent to heat-api to return to
clients, as oslo.messaging deserializes it to a more compact
exception that does not include the stack trace in the message
string.

Change-Id: I7c2da96fa07503b947c6c2c4a5f8ed3231dec9db
Closes-Bug: 1393831
Merge "Consider invalid keywords a template validation error"