lib/fs: Be more clear about invalid file names (ref #7010) (#7011)

Add specific errors for the failures, resulting in this rather than just
the generic "invalid filename":

[MRIW7] 08:50:50 INFO: Puller (folder default, item "NUL"): syncing: filename is invalid: name is reserved
[MRIW7] 08:50:50 INFO: Puller (folder default, item "fail."): syncing: filename is invalid: name ends with space or period
[MRIW7] 08:50:50 INFO: Puller (folder default, item "sup:yo"): syncing: filename is invalid: name contains reserved character
[MRIW7] 08:50:50 INFO: default: Failed to sync 3 items

lib/model/folder_sendrecv.go

@@ -337,7 +337,7 @@ func (f *sendReceiveFolder) processNeeded(snap *db.Snapshot, dbUpdateChan chan<-
 			l.Debugln(f, "Handling ignored file", file)
 			dbUpdateChan <- dbUpdateJob{file, dbUpdateInvalidate}
 
-		case runtime.GOOS == "windows" && fs.WindowsInvalidFilename(file.Name):
+		case runtime.GOOS == "windows" && fs.WindowsInvalidFilename(file.Name) != nil:
 			if file.IsDeleted() {
 				// Just pretend we deleted it, no reason to create an error
 				// about a deleted file that we can't have anyway.
@@ -345,8 +345,9 @@ func (f *sendReceiveFolder) processNeeded(snap *db.Snapshot, dbUpdateChan chan<-
 				// ignored at some point.
 				dbUpdateChan <- dbUpdateJob{file, dbUpdateDeleteFile}
 			} else {
-				// We can't pull an invalid file.
-				f.newPullError(file.Name, fs.ErrInvalidFilename)
+				// We can't pull an invalid file. Grab the error again since
+				// we couldn't assign it directly in the case clause.
+				f.newPullError(file.Name, fs.WindowsInvalidFilename(file.Name))
 				// No reason to retry for this
 				changed--
 			}