Importing the library without a world open should behave more clearly

[avdstaaij]

Moved from nikigawlik/gdmc_http_client_python#69, by @caranha:

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If you import sub-modules from the library (for example, GEO) without a minecraft world open, a large scary error message shows.

Either you should be amble to import sub-modules without a world running, or if that is impossible, a better error message (i.e "Don't forget to open the minecraft world before loading this module!") should be displayed (or maybe an appropriate error should be thrown?)