MINOR: jwt: jwt_verify returns negative values in case of error

MINOR: jwt: jwt_verify returns negative values in case of error

In order for all the error return values to be distributed on the same
side (instead of surrounding the success error code), the return values
for errors other than a simple verification failure are switched to
negative values. This way the result of the jwt_verify converter can be
compared strictly to 1 as well relative to 0 (any <= 0 return value is
an error).
The documentation was also modified to discourage conversion of the
return value into a boolean (which would definitely not work).