[core] throw an error when unable to get a connection

When a wrong URL is provided, it can happen that newConnection() returns
null. If this happens during configure_connection, the whole process is
called over and over again and results in a stack overflow.
Instead, just throw an error.

Noticed because a new test in Rails master tries to connect with
"mysql2:///wait_timeout=60", an URL unknown to JDBC.

Author: dr-itz

src/java/arjdbc/jdbc/RubyJdbcConnection.java

@@ -727,7 +727,10 @@ public synchronized IRubyObject reconnect(final ThreadContext context) {
 
     private void connectImpl(final boolean forceConnection) throws SQLException {
         setConnection( forceConnection ? newConnection() : null );
-        if ( forceConnection ) configureConnection();
+        if (forceConnection) {
+            if (getConnectionImpl() == null) throw new SQLException("Didn't get a connection. Wrong URL?");
+            configureConnection();
+        }
     }
 
     @JRubyMethod(name = "read_only?")