Add source of unwind when fatal error.

Sometimes the only log left is the errors messages. This means that
the information about why the unwind part of debuggerd is being called.
Therefore, add a little bit of extra information in the error message
to indicate why the unwind was triggered.

Bug: 377050125

Test: Forced the exec of the crash dump to fail and verified that the
Test: message is crash for a crash and unwind for debuggerd -b <PID>
Test: and debuggerd <PID>.
Change-Id: I0632ed9118c79caf4dabe6f174b25066fa9058fc