Create ErrJWTInvalid to allow modify error when JWT is invalid

[juanbelieni]

I was missing an option to change the message and code when JWT is invalid. So i created this variable that can be modified with a custom message and code.

[codecov]

Codecov Report

Merging #1627 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1627   +/-   ##
=======================================
  Coverage   85.08%   85.08%           
=======================================
  Files          28       28           
  Lines        2185     2185           
=======================================
  Hits         1859     1859           
  Misses        212      212           
  Partials      114      114           

Impacted Files	Coverage Δ
middleware/jwt.go	76.84% <100.00%> (ø)

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[FatalReadError]

See also #1552

[lammel]

Although the HTTPErrorHandler could be used for this (as described in #1552) IMHO this PR improves code readability and consistency.

Thanks @juanbelieni