Check for and remove illegal chars from error descriptions

[Vladimir Dzhuvinov]

FYI: https://tools.ietf.org/html/rfc6749#section-5.2

Values for the "error" parameter MUST NOT include characters outside the 
set %x20-21 / %x23-5B / %x5D-7E.

Values for the "error_description" parameter MUST NOT include characters 
outside the set %x20-21 / %x23-5B / %x5D-7E.

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[Yavor Vasilev]

Fixed: 75b36faa