Fix err shadowing bug that caused panics for unreachable repos

deduce.go

@@ -655,7 +655,7 @@ func (sm *SourceMgr) deduceFromPath(path string) (deductionFuture, error) {
 			defer close(c)
 			// make sure the metadata future is finished (without errors), thus
 			// guaranteeing that ru and vcs will be populated
-			_, err := root()
+			_, err = root()
 			if err != nil {
 				return
 			}
@@ -683,7 +683,6 @@ func (sm *SourceMgr) deduceFromPath(path string) (deductionFuture, error) {
 			return src, ident, err
 		}
 	}
-
 	return deductionFuture{
 		rslow: true,
 		root:  root,

manager_test.go

@@ -850,3 +850,17 @@ func TestSignalHandling(t *testing.T) {
 	}
 	clean()
 }
+
+func TestUnreachableSource(t *testing.T) {
+	// If a git remote is unreachable (maybe the server is only accessible behind a VPN, or
+	// something), we should return a clear error, not a panic.
+
+	sm, clean := mkNaiveSM(t)
+	defer clean()
+
+	id := mkPI("golang.org/notareal/repo").normalize()
+	_, err := sm.ListVersions(id)
+	if err == nil {
+		t.Error("expected err when listing versions of a bogus source, but got nil")
+	}
+}