Add option for not deleting dump files when Throttle is not available

[yarik2720]

When Throttle is not available (f.e. responds with 502 error) dump files removes and you can't analyze crash info. Log:

Presubmit failed: The requested URL returned error: 502 (22)
Failed to upload crash dump: The requested URL returned error: 502 (22)
Presubmit failed: The requested URL returned error: 502 (22)
Failed to upload crash dump: The requested URL returned error: 500 (22)

Add option on config to set not removing dump files on upload error, also there can be additional option to set maximum date for holding this files (for example 14 days)

[asherkin]

Any crashes worth analysing are happening repeatedly, there's little value in keeping every one.