Check for non-nil error in FindTemplates

[relvacode]

Resolves #599

The fmt command now correctly handles the error without panicing

$ templ fmt "this path does not exist"
(✗)  [ error=lstat this path does not exist: no such file or directory ]
(✓) Format complete [ count=1 errors=1 duration=207.096µs ]

[a-h]

Thanks!