[fuchsia] Distinguish exit-by-signal from normal exit in runner scripts.

Negative |Popen.returncode| values indicate that the process was
terminated by a signal (e.g. SIGSEGV) under POSIX systems.

Check for -ve |returncode| values and report them with a different
message text, to make it clearer when things are crashing.

Change-Id: I84a3e30d5babea9d630347e10fbd5b21df5f0706
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2213539
Auto-Submit: Wez <wez@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772028}

build/fuchsia/emu_target.py

@@ -92,15 +92,18 @@ class EmuTarget(target.Target):
       logging.error('%s did not start' % (self._GetEmulatorName()))
       return
     returncode = self._emu_process.poll()
-    if returncode:
-      logging.error('%s quit unexpectedly with exit code %d' %
-                    (self._GetEmulatorName(), returncode))
+    if returncode == None:
+      logging.info('Shutting down %s' % (self._GetEmulatorName()))
+      self._emu_process.kill()
     elif returncode == 0:
       logging.info('%s quit unexpectedly without errors' %
                    self._GetEmulatorName())
+    elif returncode < 0:
+      logging.error('%s was terminated by signal %d' %
+                    (self._GetEmulatorName(), -returncode))
     else:
-      logging.info('Shutting down %s' % (self._GetEmulatorName()))
-      self._emu_process.kill()
+      logging.error('%s quit unexpectedly with exit code %d' %
+                    (self._GetEmulatorName(), returncode))
 
   def _IsEmuStillRunning(self):
     if not self._emu_process: