mvn scm:validate does not validate the developerConnection in the pom

There are a couple of reasons for this, in ValidateMojo:

1. The check to see if the scmDeveloperConnection is not null is done on the scmConnection
2. The @parameter definitions are wrong for both the the scmConnection and scmDeveloperConnection

The check for scmConnection is never run either because of of the wrong @parameter definitions. But the actual value from the pom is normally checked anyway because the default-value for connectionUrl in AbstractScmMojo is $

. But if you run it like this:
mvn scm:validate -DconnectionUrl=invalidScmUrl
the value of pom.scm.connection will not be checked.

The supplied patch fixes these problems and logs an extra error the tells the user the value of the invalid scm connection url, to help resolve the problem.