Print a failed command's error once on stderr (closes #433)

A command that failed after startup wrote the error twice on stderr:
as an ERROR log record and as the final ERROR: line. The failure
handlers now log the failure at info level, so the record appears only
under --verbose or --debug, next to the records that led up to it.

A failed blob upload is logged at info level for the same reason. A
restore that stops on a file it cannot restore no longer logs that
file's error, which the final line carries; under --skip-errors it is
still logged.

Model: opus-5-5

internal/snapshot/upload_cancel_log_test.go

@@ -37,9 +37,9 @@ func uploadTestBlob(ctx context.Context, store storage.Storer) error {
 }
 
 // captureLog runs fn and returns what the logger wrote to stderr
-// meanwhile. The logger writes to the os.Stderr it finds when it is
-// initialized, so os.Stderr is pointed at a file first. Not
-// parallel-safe: os.Stderr and the logger are process-global.
+// meanwhile, at the --verbose level. The logger writes to the os.Stderr
+// it finds when it is initialized, so os.Stderr is pointed at a file
+// first. Not parallel-safe: os.Stderr and the logger are process-global.
 func captureLog(t *testing.T, fn func()) string {
 	t.Helper()
 
@@ -53,7 +53,7 @@ func captureLog(t *testing.T, fn func()) string {
 	previous := os.Stderr
 	os.Stderr = stderrFile
 
-	log.Initialize(log.Config{})
+	log.Initialize(log.Config{Verbose: true})
 
 	fn()
 
@@ -107,10 +107,10 @@ func TestInterruptedUploadIsNotLoggedAsError(t *testing.T) {
 	}
 }
 
-// An upload that fails for any other reason is still logged as an error.
+// An upload that fails for any other reason is still logged.
 //
 //nolint:paralleltest // captureLog replaces os.Stderr
-func TestFailedUploadIsLoggedAsError(t *testing.T) {
+func TestFailedUploadIsLogged(t *testing.T) {
 	inner, err := storage.NewFileStorer(t.TempDir())
 	if err != nil {
 		t.Fatal(err)