Bug 1694525 - End browsertime test process when error encountered; r=perftest-reviewers,sparky

Currently, raising an exception in a mozprocess callback is not guaranteed to
end the managed process; after bug 1694275, the exception will be reported but
otherwise suppressed.
Instead of raising an exception, report the error and end the test process.

Differential Revision: https://phabricator.services.mozilla.com/D106538

testing/raptor/raptor/browsertime/base.py

@@ -415,8 +415,8 @@ class Browsertime(Perftest):
                 level = level.lower()
                 if "error" in level:
                     self.browsertime_failure = msg
-                    # Raising this kills mozprocess
-                    raise Exception("Browsertime failed to run")
+                    LOG.error("Browsertime failed to run")
+                    proc.kill()
                 elif "warning" in level:
                     LOG.warning(msg)
                 else: