WhoisDomainNotFoundError and WhoisUnknownDateFormatError

[Misiu]

I've added two errors.
When a domain is not found in the registry we throw WhoisDomainNotFoundError. This error extends WhoisError, which extends PywhoisError for backward compatibility.
WhoisUnknownDateFormatError is thrown when we can't parse dates.
I've also added data_preprocessor as an optional parameter to WhoisEntry class. This way we can preprocess date strings before they are passed to the parser.
For example, for .cl domains we remove the timezone using:

lambda x: x.replace(" CLST", "").replace(" CLT", "")

[richardpenman]

Looks good! Ideally there would be some tests that cover the exception types - maybe could do that when add support for the other exception types?

[Misiu]

I can try, but as mentioned, I'm still new to Python.
Mind creating the first test showing how to test throwing exceptions, this way I'll try to add new error types with tests

[richardpenman]

Here is an example showing how to test for an exception in a unit test:
https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertRaises

[Misiu]

@richardpenman I did something like this in #268