readelf: harden against invalid input.

I also promised to fix readelf. Where in file(1) I made no attempt to
say what was bad (or even to change `goto bad` to explicitly say that
*anything* was bad), I believe that readelf is much more likely to be
shown invalid ELF files, and that it would be useful to have some clue
as to what's wrong. Relatedly, this patch removes all existing
error_exit() calls in case it's being used on multiple files.

Again, this survived ~24hrs of AFL++ trying to blow its house down.

Test: ~/AFLplusplus/afl-fuzz -i tests/files/elf -o fuzz-out -- ./readelf -a @@

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