buildman: Tidy up reporting of a toolchain error

Provide the text of the exception when something goes wrong.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/buildman/builderthread.py

@@ -457,18 +457,13 @@ class BuilderThread(threading.Thread):
                 except ValueError as err:
                     result.return_code = 10
                     result.stdout = ''
-                    result.stderr = str(err)
-                    # TODO(sjg@chromium.org): This gets swallowed, but needs
-                    # to be reported.
+                    result.stderr = f'Tool chain error for {brd.arch}: {str(err)}'
 
             if self.toolchain:
                 commit = self._checkout(commit_upto, work_dir)
                 result, do_config = self._config_and_build(
                     commit_upto, brd, work_dir, do_config, config_only,
                     adjust_cfg, commit, out_dir, out_rel_dir, result)
-            else:
-                result.return_code = 1
-                result.stderr = f'No tool chain for {brd.arch}\n'
             result.already_done = False
 
         result.toolchain = self.toolchain

tools/buildman/func_test.py

@@ -511,8 +511,10 @@ Some images are invalid'''
                 if brd.arch != 'sandbox':
                   errfile = self._builder.get_err_file(commit, brd.target)
                   fd = open(errfile)
-                  self.assertEqual(fd.readlines(),
-                          ['No tool chain for %s\n' % brd.arch])
+                  self.assertEqual(
+                      fd.readlines(),
+                      [f'Tool chain error for {brd.arch}: '
+                       f"No tool chain found for arch '{brd.arch}'"])
                   fd.close()
 
     def testBranch(self):