Debian start script does not properly check if a path does not exist or is not executable

[taylor.smock]

I've been working on getting downstreams to use our start script (with some minor patches) to hopefully avoid future issues where we get bug reports that we've fixed in the start script.

From ​https://github.com/flathub/org.openstreetmap.josm/issues/55,

➜  ~ flatpak install flathub org.openstreetmap.josm                                                       
Looking for matches…

org.openstreetmap.josm permissions:
    ipc   network   x11   file access [1]

    [1] xdg-download


        ID                              Branch          Op          Remote          Download
 1. [✓] org.openstreetmap.josm          stable          i           flathub         78.5 MB / 76.9 MB

Installation complete.
➜  ~ flatpak run org.openstreetmap.josm
which: no dpkg in (/usr/bin:/app/bin:/app/jre/bin)
Using /usr/lib/jvm/default-java/bin/java to execute josm.
/app/bin/josm: line 91: /usr/lib/jvm/default-java/bin/java: No such file or directory

This is due to source:trunk/native/linux/tested/usr/bin/josm@18297:33,41-57#L33​