bitbake: Add BBHandledException exception class

We have a problem knowing when to show the user debug information and
when not to since the code has already shown the user suitable information
about why a failure is occurring.

This patch adds a bb.BBHandledException exception class which can be used
to identify those exceptions which don't need further explanation to
the user.

This patch uses this class for the bb.providers exceptions and ensures the
command handling code correctly filters the exceptions meaning that

"bitbake invalid"

now shows an simple error message and not a python traceback.

[YOCTO #1141 partial]

(Bitbake rev: eac9249b40ae1e3aa21e016010c862664e59a8d4)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/command.py

@@ -98,9 +98,12 @@ class Command:
             else:
                 self.finishAsyncCommand("Exited with %s" % arg)
             return False
-        except Exception:
+        except Exception as exc:
             import traceback
-            self.finishAsyncCommand(traceback.format_exc())
+            if isinstance(exc, bb.BBHandledException):
+                self.finishAsyncCommand("")
+            else:
+                self.finishAsyncCommand(traceback.format_exc())
             return False
 
     def finishAsyncCommand(self, msg=None, code=None):