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.