Print errors in a friendlier way

[phseiff]

Currently, any error (caused by wrong user input or lacking internet connection) is raised as a Python Exception, including a printed message with the line number and other technical detail. This is the expected behavior when using the module from Python, but not when using it as a tool from the command line. Thus, when running main() from the if __name__ == "__main__"-clause, exceptions should be caught from outside the main()-call and printed as a string containing only the error message. In some cases, such as failing web requests, it might even be useful to let main() raise a custom, more explanatory exception than the obligatory exception provided by the python requests-library.