No data validation on get_segment_description

[TitusBraber]

I was testing some inputs in a try except block to see if the user enters a proper cronstring, during which I found something weird:

So far so good:

Your input, * * 23 * * , lead to the following repeat string: Every minute, on day 23 of the month

When you enter something like * * * facerolloverkeyboard *, or * * * * facerolloverkeyboard it will throw a FormatException as expected.

However, when you enter * * facerolloverkeyboard * *, it doesn't throw an error, instead it says:

Your input, * * facerolloverkeyboard * *, lead to the following repeat string: Every minute, on day facerolloverkeyboard of the month

I believe the path is
through: https://github.com/Salamek/cron-descriptor/blob/master/cron_descriptor/ExpressionDescriptor.py#L386
into: https://github.com/Salamek/cron-descriptor/blob/master/cron_descriptor/ExpressionDescriptor.py#L455

Similarly, when you enter facerolloverkeyboard * * * *, it doesn't throw an error, instead it says:

Your input, facerolloverkeyboard * * * *, lead to the following repeat string: At facerolloverkeyboard minutes past the hour

I believe the path is
through: https://github.com/Salamek/cron-descriptor/blob/master/cron_descriptor/ExpressionDescriptor.py#L255
into: https://github.com/Salamek/cron-descriptor/blob/master/cron_descriptor/ExpressionDescriptor.py#L455

I am not quite sure where the correct FormatException get thrown, and why it's not thrown for minutes and DoM. If you have any clue I can take it up from there and test and make a PR.