--validate explicitly tells the user that a tool definition is valid (#571)

* --validate explicitly tells the user that a tool definition is valid

* Better formatting for workflow checker warning.

Author: tetron

cwltool/main.py

@@ -898,6 +898,7 @@ def main(argsl=None,  # type: List[str]
                 return 0
 
             if args.validate:
+                _logger.info("Tool definition is valid")
                 return 0
 
             if args.pack:

cwltool/workflow.py

@@ -625,8 +625,7 @@ def static_checker(workflow_inputs, workflow_outputs, step_inputs, step_outputs)
     all_exception_msg = "\n".join(exception_msgs)
 
     if warnings:
-        _logger.warning("Workflow checker warning:")
-        _logger.warning(all_warning_msg)
+        _logger.warning("Workflow checker warning:\n%s" % all_warning_msg)
     if exceptions:
         raise validate.ValidationException(all_exception_msg)