When sending keys to the server using gpg --send-keys, if the server is
unavailable, the operation will simply fail silently, just like if the operation
succeeded.
The same thing is true for --search-keys and --recv-keys : They simply return
that search returned no result, without returning any error.
To give you an example of the consequence of this, I was tinkering with GnuPg to
create a new key pair but changed my mind about the settings for my new after
having already sent it to the server. I revoked my key and sent it again using
--send-keys...and then deleted the key pair having no use for it. I only
noticed later that the key was still valid on the server because the connection
was unavailable when I sent the revocation, and I now have no way of revoking
that public key.