u_boot_pylib: Add an exception-class for errors

Throwing an Exception is not very friendly since it is the top-level
class of all exceptions. Declare a new class instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/gitutil.py b/tools/patman/gitutil.py
index 10ea5ff..ffe0527 100644
--- a/tools/patman/gitutil.py
+++ b/tools/patman/gitutil.py
@@ -140,7 +140,7 @@
                                          'branch.%s.remote' % branch)
         merge = command.output_one_line('git', '--git-dir', git_dir, 'config',
                                         'branch.%s.merge' % branch)
-    except Exception:
+    except command.CommandExc:
         upstream, msg = guess_upstream(git_dir, branch)
         return upstream, msg