More verbose message on goma_ctl.py ensure_start failure.

For ease of investigating the issue with error reports, let me make
the error message more verbose.

Test: make goma_ctl.py raise Exception and run.
Test: I confirmed verbose error message shown.
Change-Id: I04f9a5ca4808746994c41ddce8a5b22a1975fcc2
Signed-off-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
Bug: 124218940