Added validation logic to Change editor for validity of regular expressions

This change adds input validation to the gui of Change Nodes to prevent
the user from unintentionally entering an invalid regular expression.
The user will be notified on the specific error code to help resolve
the issue.

nodes/core/logic/15-change.html

@@ -56,7 +56,18 @@
         defaults: {
             action: {value:"change",required:true},
             property: {value:"payload"},
-            from: {value:""},
+            from: {value:"",validate: function(v) {
+		        if (this.action == "change") {
+					try {
+		            	var re = new RegExp(this.from, "g");
+						return true;
+		            } catch(err) {
+						RED.notify("<strong>Error</strong>: "+err.message,"error");
+						return false;
+		            }
+		        } 
+		        return true;		        	
+			}},
             to: {value:""},
             name: {value:""}
         },