The bare tuple unpack 'executable, = re.findall(...)' raises a cryptic
ValueError if the pattern matches zero or more than one line. Replace it
with an explicit length check and a clear error message.
Also pass count=1 to str.replace() so a /newroot prefix is only stripped
once and /app/ paths are passed through unchanged.
This fixes a bug where the warning about not being a flatpaked
application was being printed for flatpaks. This was due to a
change in bwrap so that the paths no longer start with /newroot.
This is a debugging tool so there's no need for a strict sandbox. I
want to be able to extract backtraces from gdb using 'set logging on'
and it's not currently possible without using
--extra-flatpak-args="--filesystem=home".
Closes: #2313
Approved by: matthiasclasen
