Enable run on failure keyword

[robinmatz]

Hi there,

this PR is one possible solution for issue 42.

The standard procedure for running on failure in other libraries is to have your library class, in our case Mainframe3270, inherit from DynamicCore.

By overwriting DynamicCore's run_keyword function, we can take control of keyword execution by surrounding BuiltIn's run_keyword function in a try -> catch routine

def run_keyword(self, name, args, kwargs=None):
        try:
            return DynamicCore.run_keyword(self, name, args, kwargs)
        except Exception:
            self.run_on_failure()
            raise

As you can see, in the exception part, we run our own defined routine and then raise the final error.

Working with DynamicCore has one downside, which is that the main library has to have this structure:

"""Main library."""

from robotlibcore import DynamicCore

from mystuff import Library1, Library2


class MyLibrary(DynamicCore):
    """General library documentation."""

    def __init__(self):
        libraries = [Library1(), Library2()]
        DynamicCore.__init__(self, libraries)

    @keyword
    def keyword_in_main(self):
        pass

Also, library keywords must be annotated with @keyword,
see

This explains why I moved the description, as well as import arguments into __init__.py.

I understand that this is quite the change to the project structure. However, I also feel this a very useful feature supported out of the box by most popular GUI automation frameworks.

You can check the feature by running the new test cases under tests/run_on_failure/