cmd/cue: fail cue fmt if arg import path doesn't exist

CL https://cuelang.org/cl/1185292 fixed an error in which
`cue fmt` would fail if any files had invalid imports. This
was done by ignoring errors of type load.PackageError.
This introduced a bug. When running `cue fmt ./non-existing`,
the command will now succeed with status code 0 - despite the
fact that the provided import path is invalid.

To fix this we don't ignore `load.PackageError` if no additional
files were found, because in this case the import path provided
as an argument must have been invalid.

Signed-off-by: Noam Dolovich <noam.tzvi.dolovich@gmail.com>
Change-Id: I87ef60960b61924bf4c9493e5c492caa907ab294
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194124
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cmd/cue/cmd/fmt.go

@@ -73,7 +73,8 @@ func newFmtCmd(c *Command) *cobra.Command {
 			for _, inst := range builds {
 				if inst.Err != nil {
 					switch {
-					case errors.As(inst.Err, new(*load.PackageError)):
+					case errors.As(inst.Err, new(*load.PackageError)) && len(inst.BuildFiles) != 0:
+						// Ignore package errors if there are files to format.
 					case errors.As(inst.Err, new(*load.NoFilesError)):
 					default:
 						exitOnErr(cmd, inst.Err, false)

cmd/cue/cmd/testdata/script/fmt_err.txtar

@@ -1,3 +1,7 @@
+# Fail when import path provided as argument is invalid.
+! exec cue fmt ./non-existing-file
+cmp stderr non-existing-file.golden
+
 # Ignore certain package loading errors in cue fmt.
 cd issue644
 exec cue fmt x.cue
@@ -17,6 +21,8 @@ cmp x.cue out/x_cue
 cmp y.cue out/y_cue
 cd ..
 
+-- non-existing-file.golden --
+cannot find package "./non-existing-file"
 -- cue.mod/module.cue --
 module: "mod.test/x"
 -- emptypkg/.empty --