Improve error message for mistake in module compilation

[CalMacCQ]

The code snippet below is incorrect. The mistake is that in the last line we need to do module1.compile() rather than guppy.compile_module().

from guppylang import guppy, GuppyModule
from guppylang.std.quantum import qubit, h

module1 = GuppyModule(name="module1")
module1.load(qubit, h)

@guppy(module1)
def generate_plus() -> qubit:
    q = qubit()
    return h(q)

guppy.compile_module()

From the error message its not clear how to correct this

---------------------------------------------------------------------------
GuppyError                                Traceback (most recent call last)


.........

GuppyError: VarNotDefinedError(span=<ast.Name object at 0x114beab90>, _parent=None, children=[], var='qubit')