Motivation: Creating duplicate resources currently results in a 500
error being displayed by the client, with no information about the true
nature of the failure.
Modifications:
* ResourceStore now converts any errors from the etcd client into
fission errors, capturing the reason for the error in the case of a
duplicate key (as ErrorNameExists)
* ErrorNameExists errors are signaled to the client with a
409 (Conflict) HTTP status
* MakeErrorFromHTTP() now reads the body of the error response to
retrieve the actual error message instead of using the HTTP status
message
* the controller client now uses MakeErrorFromHTTP() to centralize
status code -> error code mapping
* tests for all of the resources now check that duplicate resources are
reported properly
Result: The client can now provide more context when a duplicate name is
given
related to #112
