Better error reporting, file checks

I had a segfault on running `test.sol` when it tried to import `Math.sol`, so I added a check for importing files that aren't there. This doesn't actually fix the problem, though. It just makes it a little prettier when it happens.

`report_error()` should be able to replace all calls to `log_err()` now, but it's not really a big difference which is used.