releasetools: Fix the error message.

This avoids showing a message as below:

raise BuildImageError("Failed to get tune2fs usage:\n{}".format(output))
UnboundLocalError: local variable 'output' referenced before assignment

The `output` won't be set under such a case, which unfortunately hides
the real cause. This CL addresses the issue by letting it throw
implicitly, which contains the command and outputs.

Test: `m dist`
Test: `python -m unittest test_build_image`
Change-Id: Icf015bab8869d150516246ed73552f6502127cdf