Update default merge failed error to mention CRD

The merge failure message is the same whether your change failed to
merge or it was one of its dependencies, including cross-repo
dependencies. Update the default error message for this to mention
the latter possibility so that contributors don't unnecessarily
rebase a dependent change which doesn't actually need it.

Change-Id: Ia17d9ab2a842574cf684b4ebb955dd2ec6e9ec3d
4 files changed