Don't ignore invalid operation mode silently

PR: 30669
Submitted by:	Ben Galbraith


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@276894 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/taskdefs/optional/ssh/Scp.java

@@ -176,7 +176,7 @@ public class Scp extends SSHBase {
                     upload(fromUri, toUri);
                 }
             } else if (isFromRemote && isToRemote) {
-                // not implemented yet.
+                throw new BuildException("Copying from a remote server to a remote server is not supported.");
             } else {
                 throw new BuildException("'todir' and 'file' attributes "
                     + "must have syntax like the following: "