Add manifest groups

Allows specifying a list of groups with a -g argument to repo init.
The groups act on a group= attribute specified on projects in the
manifest.
All projects are implicitly labelled with "default" unless they are
explicitly labelled "-default".
Prefixing a group with "-" removes matching projects from the list
of projects to sync.
If any non-inverted manifest groups are specified, the default label
is ignored.

Change-Id: I3a0dd7a93a8a1756205de1d03eee8c00906af0e5
Reviewed-on: https://gerrit-review.googlesource.com/34570
Reviewed-by: Shawn Pearce <sop@google.com>
Tested-by: Shawn Pearce <sop@google.com>
diff --git a/error.py b/error.py
index 812585c..78c5c0e 100644
--- a/error.py
+++ b/error.py
@@ -77,6 +77,18 @@
       return 'in current directory'
     return self.name
 
+
+class InvalidProjectGroupsError(Exception):
+  """A specified project is not suitable for the specified groups
+  """
+  def __init__(self, name=None):
+    self.name = name
+
+  def __str__(self):
+    if self.Name is None:
+      return 'in current directory'
+    return self.name
+
 class RepoChangedException(Exception):
   """Thrown if 'repo sync' results in repo updating its internal
      repo or manifest repositories.  In this special case we must