Improve error message to suggest fix

[nareddyt]

For this line:

copybara/java/com/google/copybara/transform/RegexTemplateTokens.java

Line 266 in c12c048

"Regex group is used in template multiple times: %s",

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.

[hsudhof]

This is a setting that probably warrants more documentation as it is commonly misunderstood. repeated_groups means the same character sequence can be captured by a repeated group, but this is not what many users expect (the same pattern matched several times).