Trim leading slash off of siapath provided in siac download

MERGE REQUEST

Changes to Work Flow

There was a UX bug in siac where a download would error out saying the download did not succeed but the download did actually succeed in the background. This was due to how the leading / of a siapath is handled by the API. Leading / are automatically removed so a user can upload a file with siapath /file and the actual siapath on the network will be just file. This means when a user tries to download /file, the API handles the / fine but siac downloadprogress is still looking for /file and therefore never finds it and times out.

siac no trims the / immediately for a download.

Testing Updates

verified on live node

Modules and Packages Impacted

• /cmd/siac

Close #3432 (closed)