Add a warning for invalid dictionaries

[jvoisin]

I'm not sure that displaying an error message is enough. Should we bail out like afl/libfuzzer does?

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}