Enhancement: do not open panel when window doesn't exist

[randy3k]

Another enhancement of issue #726.

Throw RuntimeError If the current view is detached, it will result in a GitSavvyError without showing panel.

cc: @asfaltboy, hope this would be the final fix.

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• 2017-08-21T01:20:05.201Z:bf0ae11: The pull request was created
• 2017-08-21T01:20:20.302Z:bf0ae11: @randy3k checked I have read and agree to the [contribution guidelines](https://github.com/divmain/GitSavvy/blob/master/CONTRIBUTING.md)..
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• 2017-08-21T01:20:23.942Z:bf0ae11: @randy3k selected patch as the semantic version.