bundle2: introduce a ``params`` attribute to BundleValueError

bundle2: introduce a ``params`` attribute to BundleValueError

We'll first use it for unsupported mandatory parameters on parts.

--- a/mercurial/error.py	Wed May 28 15:51:19 2014 -0700
+++ b/mercurial/error.py	Wed May 28 15:53:34 2014 -0700
@@ -100,13 +100,15 @@
 
 # bundle2 related errors
 class BundleValueError(ValueError):
-    """error raised when bundle2 cannot be processed
-
-    Current main usecase is unsupported part types."""
+    """error raised when bundle2 cannot be processed"""
 
-    def __init__(self, parttype):
+    def __init__(self, parttype, params=()):
         self.parttype = parttype
-        super(BundleValueError, self).__init__(parttype)
+        self.params = params
+        msg = parttype
+        if self.params:
+            msg = '%s - %s' % (msg, ', '.join(self.params))
+        super(BundleValueError, self).__init__(msg)
 
 class ReadOnlyPartError(RuntimeError):
     """error raised when code tries to alter a part being generated"""