Improve gcf-am error handling.
Improve gcf-am error handling.

Always use constants for error codes. Always log a message when
returning an error.

plantigrade/geni-tools,GENI-NSF/gram,tcmitchell/geni-tools,ahelsing/geni-tools,plantigrade/geni-tools,tcmitchell/geni-tools,GENI-NSF/gram,GENI-NSF/gram,ahelsing/geni-tools