Fixing bug when there is no error, but a linefeed or more is returned as...

When running UglifyJS filter in my Windows environment, I had a problem where the output was generated correctly, but the $Process->error() method returned two line feeds (char 10) and nothing more.

string(2)"

"

The problem occurred only with the UglifyJS filter, so I added a check to only throw the RuntimeException only when there is an error message that is not only whitespace, tabs or line feeds.