tools: download_file_from_url: improve UX and avoid cyclic dependencies

tools: download_file_from_url: improve UX and avoid cyclic dependencies

plus some refactoring

* drop worker, cannot be done here (RPCEnv is in pve-access-control)
* actually output the wrong "got" hash on mismatch
* die on existing file with mismatched
* drop double array for passing cmd
* drop `/usr/bin` prefix
* adapt rename error message
* add error handling for unlinking the temp. file

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>