Use common base class for all exceptions thrown by jwt-cpp

[sirzooro]

This was initially described in #212. Please add new class derived from std::system_error, and derive all jwt-cpp exceptions from it. This would allow to write bigger block of code, and at the end add few catch sections for different kinds of errors (e.g. network error, jwt validation error, etc.). You also could store some extra information in additional fields of exception class, e.g. name of claim which did not pass validation.

Desktop:

• OS: RedHat 8
• Compiler g++ 10.3.1
• Version jwt-cpp v0.6.0

[prince-chrismc]

Extra bit for this is some of the algo families added latter on mix other algo error codes #318 (comment)

more possible work in this direction #318 (review)