flatpak-coredumpctl: Guard against unexpected coredumpctl output

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.
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