Make get_client() infallible

If getting the system proxies fails, the request will fail.
This will enable us to use only the reqwest error type easily
in many places.
2 jobs for infallible-get-client in 37 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Check
passed #808262
check

00:01:10

 
  Test
passed #808263
flatpak
flatpak

00:36:01