gbp buildpackage was failing silently (with a return value of 1 but no
error message)
when I tried to build cargo. After adding a bunch of additional debug
prints I tracked
it down to the following exception handler in buildpackage.py

     except CommandExecFailed:
         retval = 1

Adding some extra debug output to this exception handler revealed that
the problem
was that git-buildpackage was not installed.

Please consider printing an error message in this error handling path so
the user
knows what has gone wrong. In my tests I used the following code
snippet, something
along the lines of

     except CommandExecFailed as err:
         retval = 1
         print('command execution failed err='+repr(err))