pgem_update should attempt to detect updates that will fail before beginning

[dimo414]

Original report by Michael Diamond (Bitbucket: dimo414).

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Currently pgem_update updates all gems and ProfileGem separately, which can be problematic if there's a dependency between different components and only some of them successfully update.

As a concrete example, after if fa2b72e is pulled down prompt.gem also needs to be updated to pull down dimo414/prompt.gem@7de97087f766 - if prompt.gem fails to update the resulting shell will be in a sad state (in this particular it should be limited to noisy log messages).