Improve error message to suggest fix

For this line: https://github.com/google/copybara/blob/c12c048c95bc5157f7d67ec8a9a77eeec3ba6115/java/com/google/copybara/transform/RegexTemplateTokens.java#L266

This makes it seem like copybara cannot handle repeated use of regex group. But it can, the user just needs to configure `repeated_groups = True` in `core.replace`.

Can we clarify the error message to suggest this?

If needed, I can make a quick PR.