Provide custom error message to assertion keywords

[robinmatz]

It would be a nice enhancement to be able to provide a custom error message to the keywords in assertion.py.

We have this feature in BuiltIn in Should Be Equal, Should Be True, and Length Should Be, to name a few.

We could introduce the same msg parameter to let the user provide a custom error message.

The following would be an example of this usage.

Row Count Is Equal To X    selectStatement   1    msg=my error message

I know you can work around this with Query and Length Should Be .

But wouldn't it be nice if the library provided this possibility out of the box?

[robinmatz]

Since this seems like an easy feature to implement, I can open a PR once I get to it.

[amochin]

I've just merged the pull request and copied the added tests into the new structure.
Thanks for your contribution, Robin! :)

[robinmatz]

Thanks alot for merging this. It is great to see that the development of this library has gained new momentum!