proto: Export ErrInvalidUTF8

[sshaplygin]

PR motived by issue golang/protobuf#1642

[google-cla]

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[puellanivis]

Thank you for your interest into contributing to this project.

Unfortunately this project does not accept GitHub pull requests as the source-of-truth for this project is hosted at https://go.googlesource.com/protobuf. This project page on GitHub is a mirror of that other repository.

If you would like to contribute to this project, please follow the contribution guidelines for instructions on how to send a change. If the change you'd like to make is more substantial or introduces any new features, then it should first be discussed on the issue tracker.