[MERGE] Raise NoSuchRevision from fetch, instead of InstallFailed

[Aaron Bentley]

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Robert Collins wrote:
> Well, InstallFailed is meant to be what people catch when they call
> fetch - though I guess this has drifted over time.
> 
> given we're not catching it anywhere, and we let other errors bubble up
> already....

It removes a public symbol, so it's an API break.  It should be noted in
NEWS, and we should update the min_api_version.

Aaron
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