Better error message when genomedata-load is not on the PATH

[EricR86]

Original report (archived issue) by Eric Roberts (Bitbucket: ericr86, GitHub: ericr86).

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Currently there is an error on line 79 in _load_data.py where if genomedata-load is not on your path, python attempts to pipe into a process it cannot find resulting in a "write error: Broken pipe" and "OSError: [Errno 2] No such file or directory".

There should be a better error message for this case.