PEcAn.remote::remote.copy.to and from don't check that `host` argument isn't NULL

[Eric R. Scott]

Bug Description

Made a typo and did remote.copy.to(host = settings$hots, ...) and rsync failed with uninformative error. It ended up running this:

rsync '-a' '-q' '/path/to/src' ' :/path/to/dst' 

instead of

rsync '-a' '-q' '/path/to/src' ' username@host:/path/to/dst' 

Expected behavior

PEcAn functions should fail early if host is NULL with informative message.

[Meet Agrawal]

I would like to work on this issue.