Add better debugging to `mb run`.

`mb run` (i.e., `python tools/mb.py run`) can be used to run
tests under swarming directly. However, it turns out that there
were a couple of places where we were swallowing error output,
making it hard to understand why things were failing silently :).

This CL should fix at least some of those cases, and also tweaks
the output a bit to be a bit more readable.

Change-Id: If6b412a9afad472b35220cb23721002aaaadcd32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623745
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662869}

tools/mb/mb.py

@@ -352,13 +352,17 @@ class MetaBuildWrapper(object):
     if not vals:
       return 1
     if self.args.build:
+      self.Print('')
       ret = self.Build(self.args.target)
       if ret:
         return ret
+
+    self.Print('')
     ret = self.RunGNIsolate(vals)
     if ret:
       return ret
 
+    self.Print('')
     if self.args.swarmed:
       return self._RunUnderSwarming(self.args.path, self.args.target)
     else:
@@ -450,8 +454,24 @@ class MetaBuildWrapper(object):
         '-I', isolate_server,
         '--namespace', namespace,
       ]
-    ret, out, _ = self.Run(cmd, force_verbose=False)
+
+    # Talking to the isolateserver may fail because we're not logged in.
+    # We trap the command explicitly and rewrite the error output so that
+    # the error message is actually correct for a Chromium check out.
+    self.PrintCmd(cmd, env=None)
+    ret, out, err = self.Run(cmd, force_verbose=False)
     if ret:
+      self.Print('  -> returned %d' % ret)
+      if out:
+        self.Print(out, end='')
+      if err:
+        # The swarming client will return an exit code of 2 (via
+        # argparse.ArgumentParser.error()) and print a message to indicate
+        # that auth failed, so we have to parse the message to check.
+        if (ret == 2 and 'Please login to' in err):
+          err = err.replace(' auth.py', ' tools/swarming_client/auth.py')
+          self.Print(err, end='', file=sys.stderr)
+
       return ret
 
     isolated_hash = out.splitlines()[0].split()[0]
@@ -467,6 +487,7 @@ class MetaBuildWrapper(object):
     self._AddBaseSoftware(cmd)
     if self.args.extra_args:
       cmd += ['--'] + self.args.extra_args
+    self.Print('')
     ret, _, _ = self.Run(cmd, force_verbose=True, buffer_output=False)
     return ret
 
@@ -1500,7 +1521,7 @@ class MetaBuildWrapper(object):
     if self.args.jobs:
       ninja_cmd.extend(['-j', '%d' % self.args.jobs])
     ninja_cmd.append(target)
-    ret, _, _ = self.Run(ninja_cmd, force_verbose=False, buffer_output=False)
+    ret, _, _ = self.Run(ninja_cmd, buffer_output=False)
     return ret
 
   def Run(self, cmd, env=None, force_verbose=True, buffer_output=True):