If it warns, please ensure that it still returns the input string it recieved; otherwise, the output to the shell buffer is affected.
> dirtrack.el can occasionally call a signal an error when it's regex
> matches a directory that does not exist. This is generally called when
> dirtrack-list is slightly incorrect, but this is a hard variable to get
> precise, for some prompts; in the cases where it matches a non-existent
> directory, I believe it should print the input as normal, instead of
> swallowing it and erroring.
I have changed it to use `warn' instead of `error'.