Figure out a better error message when given an unstripped binary

[njsmith]

I'm not sure what's going on with the binaries generated by mingw-w64, but by default they contain a ton of data beyond the end of that described in the section table, and that strip removes. So it would be good to figure this out so we can detect it and say "you should run strip".

Strip also removes a bunch of debug sections. I suspect that even if we handled the anonymous trailing data described above, these debug sections might still be something of a problem, because strip may not be able to remove a section from the middle of a file while keeping a later section. (OTOH maybe it can, who knows.)

Also need to check how this works for MSVC-generated binaries and their version of stripping.