Otherwise, one can't run "go fmt" on a directory containing Go files if
none of them are buildable (e.g. because of build tags). This is
counter-intuitive, as fmt will format all Go files anyway.

If we encounter such a load error, ignore it and carry on. All other
load errors, such as when a package can't be found, should still be
shown to the user.

Add a test for the two kinds of load errors. Use fmt -n so that any
changes to the formatting of the files in testdata don't actually get
applied. The load errors still occur with -n, so the test does its job.

Fixes #22183.

Change-Id: I99d0c0cdd29015b6a3f5286a9bbff50757c78e0d
Reviewed-on: https://go-review.googlesource.com/75930
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>