Bug 1129394 - produce a useful error message for |mach build-backend| without having run configure; r=ted.mielczarek

Checking for config.status ourselves avoids the unpleasant situation of
the subprocess module checking for us and producing a cryptic error
message.

python/mozbuild/mozbuild/mach_commands.py

@@ -586,6 +586,12 @@ class Build(MachCommandBase):
         python = self.virtualenv_manager.python_path
         config_status = os.path.join(self.topobjdir, 'config.status')
 
+        if not os.path.exists(config_status):
+            print('config.status not found.  Please run |mach configure| '
+                  'or |mach build| prior to building the %s build backend.'
+                  % backend)
+            return 1
+
         args = [python, config_status, '--backend=%s' % backend]
         if diff:
             args.append('--diff')