emerge: display short EROFS message (490732)

This patch causes emerge to display a short "Read-only file system:
'/foo/bar'" message instead of a full traceback. The file path in the
new message should be enough information to diagnose the reason for
the error, so the full traceback is excessively verbose in comparison.

X-Gentoo-Bug: 490732
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=490732