diff -r 1e497df514e2 -r 931a72e00efa mercurial/error.py
--- a/mercurial/error.py	Thu Feb 10 09:03:06 2011 +0100
+++ b/mercurial/error.py	Fri Feb 18 20:25:25 2011 +0100
@@ -51,6 +51,10 @@
 class CapabilityError(RepoError):
     pass
 
+class RequirementError(RepoError):
+    """Exception raised if .hg/requires has an unknown entry."""
+    pass
+
 class LockError(IOError):
     def __init__(self, errno, strerror, filename, desc):
         IOError.__init__(self, errno, strerror, filename)