Tone down range position errors from fatal error to a positioned warning

[retronym]

Suggestion from a customer trying to implement code coverage in a large codebase using a compiler plugin that enabled range positions. Makes sense to me.

/cc @adriaanm in case you're in range-pos land in the REPL adventures.

[adriaanm]

Good idea -- I'm not there yet, but will keep in mind.

[jvican]

I agree this is desirable. It could help Scalameta and Scalafix too (/cc @xeno-by @olafurpg), as well as tooling using range positions for auxiliary features.

[olafurpg]

This would be nice. The Scalameta Semantic API, which Scalafix uses, requires -Yrangepos. I regularly hit on assertion errors when trying out -Yrangepos on random projects. A warning would be preferable.

[SethTisue]

The Scalameta Semantic API, which Scalafix uses, requires -Yrangepos

#472 will interest you, then

[SethTisue]

@hrhino is interested in -Yrangepos now...