Fix GET-PROTOCOL-BY-NAME with unknown protocols
The function SB-BSD-SOCKETS:GET-PROTOCOL-BY-NAME did not signal an error when it was unable to find a protocol. This is fixed now and an error of type UNKNOWN-PROTOCOL, with an accessor named UNKNOWN-PROTOCOL-NAME, is being signaled. These two names are also exported from the package SB-BSD-SOCKETS. A test case is added. Several other test cases are fixed with regard to :NO-ERROR clauses inside HANDLER-CASEs.