cmd/compile: report profile open/parse errors

cmd/compile: report profile open/parse errors

Currently we fail to print errors from os.Open and profile.Parse of the
PGO profile, losing context useful to understand these errors.

In fixing this, cleanup error use overall to return an error from
pgo.New and report the problematic file at the top level.

Change-Id: Id7e9c781c4b8520eee96b6f5fe8a0b757d947f7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/474995
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>