Add more robust error handling of BoundingBox inputs

[hollandjg]

          What do you think about using a try/except block here in case a bad representation of a bbox, such as "1, 2, 3, 4, 5" or something that cannot be cast as int, is passed to the constructor?

I tried with these bad inputs:

>>> BoundingBoxParser.convert('1,2,3,"a"')
BoundingBox(x1=1, y1=2, x2=3, y2='a')

>>> BoundingBoxParser.convert('1,2,3,4,5')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "ebfloeseg\bbox.py", line 74, in convert     
    value = BoundingBox(*raw_value)
            ^^^^^^^^^^^^^^^^^^^^^^^
TypeError: BoundingBox.__new__() takes 5 positional arguments but 6 were given

In the former something that's not numeric was accepted, and in the latter the error message is somewhat misleading.

Originally posted by @cpaniaguam in #60 (comment)