Use DOCKER_ERROR for docker errors

src/boxerror.js

@@ -18,16 +18,17 @@ function BoxError(reason, errorOrMessage, details) {
 
     this.name = this.constructor.name;
     this.reason = reason;
+    this.details = details || {};
+
     if (typeof errorOrMessage === 'undefined') {
         this.message = reason;
     } else if (typeof errorOrMessage === 'string') {
         this.message = errorOrMessage;
-    } else {
-        this.message = 'Internal error';
+    } else { // error object
+        this.message = errorOrMessage.message;
         this.nestedError = errorOrMessage;
+        _.extend(this.details, errorOrMessage); // copy enumerable properies
     }
-
-    this.details = details || {};
 }
 util.inherits(BoxError, Error);
 BoxError.ACCESS_DENIED = 'Access Denied';