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
f3fc62c8a0