Precipitation downloader needs to remove request from queue if interrupted

[barneydobson]

If you mistakenly submit a bad request for precipitation downloader (e.g., wrong dates, bad CRS, python crashes) - it will stay in your CDS queue and block other downloads. I think it would crash eventually.. but I don't know how long that will take. I assume the API lets you cancel requests

[barneydobson]

Changing this to a bug, as the CDS downloader currently is super unreliable as they are migrating their ... something or other. See https://forum.ecmwf.int/t/a-new-cds-soon-to-be-launched-expect-some-disruptions/1607

Linked with #23 as @cheginit 's suggestion may be preferrable.

Also linked with #55 - if we are happy with using a design storm, then this is definitely a low priority