config: check config names more carefully and report errors - fixes #3506

Before this change it was possible to make a remote with an invalid
name in the config file, either manually or with `rclone config
create` (but not with `rclone config`).

When this remote was used, because it was invalid, rclone would
presume this remote name was a local directory for a very suprising
user experience!

This change checks remote names more carefully and returns errors
- when the user tries to use an invalid remote name on the command line
- when an invalid remote name is used in `rclone config create/update/password`
- when the user tries to enter an invalid remote name in `rclone config`

This does not prevent the user entering a remote name with invalid
characters in the config manually, but such a remote will fail
immediately when it is used on the command line.

cmd/cmd.go

@@ -174,7 +174,11 @@ func NewFsSrcDstFiles(args []string) (fsrc fs.Fs, srcFileName string, fdst fs.Fs
 	// If file exists then srcFileName != "", however if the file
 	// doesn't exist then we assume it is a directory...
 	if srcFileName != "" {
-		dstRemote, dstFileName = fspath.Split(dstRemote)
+		var err error
+		dstRemote, dstFileName, err = fspath.Split(dstRemote)
+		if err != nil {
+			log.Fatalf("Parsing %q failed: %v", args[1], err)
+		}
 		if dstRemote == "" {
 			dstRemote = "."
 		}
@@ -197,7 +201,10 @@ func NewFsSrcDstFiles(args []string) (fsrc fs.Fs, srcFileName string, fdst fs.Fs
 
 // NewFsDstFile creates a new dst fs with a destination file name from the arguments
 func NewFsDstFile(args []string) (fdst fs.Fs, dstFileName string) {
-	dstRemote, dstFileName := fspath.Split(args[0])
+	dstRemote, dstFileName, err := fspath.Split(args[0])
+	if err != nil {
+		log.Fatalf("Parsing %q failed: %v", args[0], err)
+	}
 	if dstRemote == "" {
 		dstRemote = "."
 	}