When the source jbod is not in mxmounts, we currenty get

    **ERROR: unknown jbod: X2083
    Use of uninitialized value in concatenation (.) or string at /usr/sbin/mxmirror line 410, <F> line 240.
    Use of uninitialized value in concatenation (.) or string at /usr/sbin/mxmirror line 410, <F> line 240.
    **ERROR: CAN'T PARSE FROM/TO: X2083 X2076

because the `undef` result of `convert_to_sourcepath` is
used by `convert_to_destinationpath`.

Bark out early when the source path is not parsable.

This changes output to

    buczek@pummelfee:~/git/mxtools (master)$ mxmirror/mxmirror
    **ERROR: unknown jbod: X2083
    **ERROR: CAN'T PARSE FROM: X2083 X2076