Doubly-nested errors when using context manager

[brendan-ward]

When using the context manager with Map(...) as map:, if there is an error in setting map properties or rendering map, the errors get raised twice:

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File <file>, line 296, in <module>
    png = map.renderPNG()
TypeError: __exit__(): incompatible function arguments. The following argument types are supported:
    1. (self: pymgl._pymgl.Map, arg0: capsule, arg1: capsule, arg2: capsule) -> None