Keep validation warnings on stderr
Keep validation warnings on stderr

This ensures that all validation warnings are emitted on stderr and not stdout.

Using stdout makes it impossible to write a command that emits formatted output. For example, `embroider-compat-audit --json > results.json` is supposed to dump a json summary of whether your app has any embroider compatibility problems. But right now, those results get ember-svg-jar warnings mixed into the json, breaking the formatting.

`console-ui` has weird opinions about stderr vs stdout. It seems to think that things at log level `ERROR` go on stderr, but things on log level `WARNING` go on stdout. This is backwards, because warnings shouldn't disrupt the normal program output, while *maybe* sometimes error should.

ivanvotti/ember-svg-jar,ivanvotti/ember-svg-jar,ivanvotti/ember-svg-jar