KaJaSim feature idea: add expected syntax to error message, and correct typo

[hmedina]

Currently, when one writes an ill-formated statement, an error is shown saying "syntax error". It would be useful if the message were accompanied by what the expected syntax was. For example, a line like:

%init: A() 10

Is accompanied by an error message:

Error at line X, characters 0-9: : Malformed initial condition

The error message could include the syntax definition, in plain English or in the manual's technical terms.

Error at line X, characters 0-9: Malformed initial condition; syntax for initial conditions is "%init: [kappa expression] [initial value]"

Also, there seems to be a typo with the double colon between the character identifiers and the error message.