Add explicit message for unsupported binary signature

Verifying the content of Release.gpg made us fail on binary signatures
which were never officially supported (apt-secure manpage only documents
only the generation of ASCII armored), but silently accepted by gpgv as
we passed it on unchecked before.

The binary format is complex and is itself split into old and new
formats so adding support for this would not only add lots of code but
also a good opportunity for bugs and dubious benefit.

Reporting this issue explicitly should help repository creators figure
out the problem faster than the default NODATA message hinting at
captive portals.

Given that the binary format has no file magic or any other clear and
simple indication that this is a detached signature we guess based on
the first two bits only – and by that only supporting the "old" binary
format which seems to be the only one generated by gnupg in this case.

References: e2965b0b6bdd68ffcad0e06d11755412a7e16e50
Closes: #921685

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