Produce nice error message when .exe submitted instead of source code

[tom93]

Currently, if a user tries to submits and uploads their compiled .exe instead of their source file, it causes a 500 Internal Server Error (invalid byte sequence in UTF-8). We should instead display a helpful error message. (Note that this error could also happen if the source file has the wrong encoding.)

Log:

Completed 500 Internal Server Error in 112ms (ActiveRecord: 58.2ms)
invalid byte sequence in UTF-8 excluded from capture: DSN not set

ArgumentError (invalid byte sequence in UTF-8):
  app/controllers/problems_controller.rb:90:in `block in submit'
  app/controllers/problems_controller.rb:89:in `submit'


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