Improve error message if the `purs` binary's dynamic loader does not exist

[hdgarrood]

For systems which aren't based on glibc, such as Alpine, or for systems like Nix which put the glibc dynamic loader in a weird place, the installer fails with this rather unhelpful error:

✖ Verify the prebuilt binary works correctly
  /home/skykanin/.npm-global/lib/node_modules/purescript/purs.bin --version
  Error: spawn /home/skykanin/.npm-global/lib/node_modules/purescript/purs.bin ENOENT
    at Process.ChildProcess._handle.onexit (internal/child_process.js:268:19)

This was observed in both #16 and #21. I would like to improve this error by catching ENOENT, checking that purs.bin does in fact exist (just in case), and if it does, checking whether its dynamic loader is a file which exists. If it doesn't, we could output a message along the lines of:

The prebuilt binary does not appear to be suitable for your system (specifically, the dynamic loader /lib64/ld-linux-x86-64.so.2 was not found).

and then fall back to building from source. We'll have to depend on an ELF parser to manage this but I think it's justifiable.