"file not found" error message should be friendlier

[catamorphism]

If I do:
$ rustc some-file-that-doesnt-exist.rs

I get:

upcall fail '.t144 != 0u', lib/io.rs:110

Indeed, the file_reader function in lib/io.rs fails with an uncaught exception if the file that's being opened doesn't exist. It would be better to print a more informative error message or to fail in a non-fatal way.

[brson]

This has been fixed. Now says 'error opening some-file-that-doesnt-exist.rs'