cmd/go: allow go fmt to complete when embedded file is missing

Fixes #43273

Change-Id: I75fe2e608cb43c048e3c2a22fe7fbb6eb779504a
Reviewed-on: https://go-review.googlesource.com/c/go/+/280452
Trust: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/fmtcmd/fmt.go

@@ -75,7 +75,8 @@ func runFmt(ctx context.Context, cmd *base.Command, args []string) {
 		}
 		if pkg.Error != nil {
 			var nogo *load.NoGoError
-			if errors.As(pkg.Error, &nogo) && len(pkg.InternalAllGoFiles()) > 0 {
+			var embed *load.EmbedError
+			if (errors.As(pkg.Error, &nogo) || errors.As(pkg.Error, &embed)) && len(pkg.InternalAllGoFiles()) > 0 {
 				// Skip this error, as we will format
 				// all files regardless.
 			} else {

src/cmd/go/testdata/script/embed_fmt.txt

@@ -0,0 +1,22 @@
+# go fmt ignores file not found
+go fmt xnofmt.go
+cmp xnofmt.go xfmt.ref
+! go build xnofmt.go
+stderr 'xnofmt.go:5:12: pattern missing.txt: no matching files found'
+
+-- xnofmt.go --
+package p
+
+import "embed"
+
+//go:embed missing.txt
+var X  embed.FS
+-- xfmt.ref --
+package p
+
+import "embed"
+
+//go:embed missing.txt
+var X embed.FS
+-- go.mod --
+module m