Add EmitError to mockEmitter

Adheres to Emitter interface in loggregatorlib:
 https://github.com/cloudfoundry/loggregatorlib/commit/b02c84bd8b7e8ad3658e264cae02e2fd16d29e99

Signed-off-by: Zach Robinson <zrobinson@pivotallabs.com>
Auto-merge remote-tracking branch 'upstream/master'