Display an error message when submitting executable.

[BelgianSalamander]

This pull request should resolve #257.

It displays an error message either when a submission's source has an invalid text encoding or the source contains a null character. Either conditions would have previously caused a 500 Internal Server Error.

• Having an invalid text encoding caused verification to throw an error when it tried checking if the source is blank (String#blank raises an exception when called on a string with an invalid encoding). This means that this needs to be checked before running any verification.
• If the source was valid UTF-8 but contained a null character, an error would be raised when trying to save the record because PostgreSQL does not support having null characters in strings.

[coveralls]

coverage: 37.531% (+0.008%) from 37.523%
when pulling e06d528 on BelgianSalamander:master
into 51571c5 on NZOI:master.