gui, lib/ignore: Handle editing ignores with error (fixes #5425) (#6757)

This changes the error handling in loading ignores slightly:

- There is a new ParseError type that is returned as the error
  (somewhere in the chain) when the problem was not an I/O error loading
  the file, but some issue with the contents.

- If the file was read successfully but not parsed successfully we still
  return the lines read (in addition to nil patterns and a ParseError).

- In the API, if the error IsParseError then we return a successful
  HTTP response with the lines and the actual error included in the JSON
  object.

- In the GUI, as long as the HTTP call to load the ignores was
  successful we can edit the ignores. If there was an error we show this
  as a validation error on the dialog.

Also some cleanup on the Javascript side as it for some reason used
jQuery instead of Angular for this editor...

lib/ignore/ignore_test.go

@@ -195,6 +195,9 @@ func TestBadPatterns(t *testing.T) {
 		if err == nil {
 			t.Errorf("No error for pattern %q", pat)
 		}
+		if !IsParseError(err) {
+			t.Error("Should have been a parse error:", err)
+		}
 	}
 }
 
@@ -1006,10 +1009,13 @@ func TestIssue4901(t *testing.T) {
 	for i := 0; i < 2; i++ {
 		err := pats.Load(".stignore")
 		if err == nil {
-			t.Fatalf("expected an error")
+			t.Fatal("expected an error")
 		}
 		if fs.IsNotExist(err) {
-			t.Fatalf("unexpected error type")
+			t.Fatal("unexpected error type")
+		}
+		if !IsParseError(err) {
+			t.Fatal("failure to load included file should be a parse error")
 		}
 	}
 
@@ -1094,9 +1100,13 @@ func TestPartialIncludeLine(t *testing.T) {
 	}
 
 	for _, tc := range cases {
-		if err := pats.Parse(bytes.NewBufferString(tc), ".stignore"); err == nil {
+		err := pats.Parse(bytes.NewBufferString(tc), ".stignore")
+		if err == nil {
 			t.Fatal("should error out")
 		}
+		if !IsParseError(err) {
+			t.Fatal("failure to load included file should be a parse error")
+		}
 	}
 }
 
@@ -1177,5 +1187,8 @@ func TestEmptyPatterns(t *testing.T) {
 		if err == nil {
 			t.Error("Should reject invalid pattern", tc)
 		}
+		if !IsParseError(err) {
+			t.Fatal("bad pattern should be a parse error")
+		}
 	}
 }