fix: error on file arguments that match nothing

approvals/kir_test.TestCLI.MissingFile.exitcode.approved.txt

@@ -1 +1 @@
-0
+1

approvals/kir_test.TestCLI.MissingFile.stderr.approved.txt

@@ -0,0 +1 @@
+error: no such file or match for "does-not-exist.yaml"

fileutil/fileutil.go

@@ -1,6 +1,7 @@
 package fileutil
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 )
@@ -14,6 +15,14 @@ func FindFiles(args []string) ([]string, error) {
 			return nil, err
 		}
 
+		// filepath.Glob returns no matches and no error both for a glob
+		// pattern that matches nothing and for a literal path that does not
+		// exist. Either way the argument named nothing, which is almost
+		// always a typo; surface it instead of silently skipping it.
+		if len(matchedFiles) == 0 {
+			return nil, fmt.Errorf("no such file or match for %q", filePath)
+		}
+
 		for _, matchedFile := range matchedFiles {
 			fileInfo, err := os.Stat(matchedFile)
 			if err != nil {

fileutil/fileutil_test.go

@@ -67,3 +67,12 @@ func TestFindFiles(t *testing.T) {
 		})
 	}
 }
+
+// A path that names nothing (a typo'd or missing file) must be reported as an
+// error rather than silently ignored.
+func TestFindFilesMissing(t *testing.T) {
+	missing := filepath.Join(t.TempDir(), "does-not-exist.yaml")
+	if _, err := FindFiles([]string{missing}); err == nil {
+		t.Fatalf("FindFiles(%q) returned nil error for a missing path", missing)
+	}
+}