Set a better error msg for non uft8 encoded files

[jecaro]

fix #5683

When the file doesn't contain the expected header, I first check if the file content is UTF-8 encoded. Depending on the result, I show a different message. For example, running roc on a binary file leads to this error:

── MISSING HEADER ───────────────────────────────────────────────── ./test.bin ─

I am expecting a header, but the file is not UTF-8 encoded.

I am expecting a module keyword next, one of interface, app, package
or platform.

[Anton-4]

Looks good, thanks @jecaro!