parser should warn and exit when undeclared variables are used

[Jochem Snuverink]

See title. Right now the following works in the parser:

a = 1;
c = a * b; // c will be 0 since b is not declared before and initialised to 0

it would be better if an error is given for the non-declared b.

[Jochem Snuverink]

This has been fixed. The code will exit.

Furthermore if a variable is redefined when a previous value existed a warning is given.