Improve error message when trying to encode unsupported object type

[kerrick-lyft]

When you try to encode an unsupported object type, you can get a hard-to-understand error:

>>> import numpy
>>> import ujson
>>> ujson.dumps(numpy.bool_(True))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: Maximum recursion level reached

Add a hint to this error message about why the encoding may have failed: (maybe you tried encoding a type of object that isn't supported?)

Fixes #339

[hugovk]

Hello, thank you very much for the issue and PR.

Since UltraJSON 2.0 was released with 53f85b1, this is no longer valid:

Removed generic serialization of objects/iterables

The behavior of ujson has always been to try to serialize all objects in
any way possible. This has been quite a deviation from other json
libraries, including Pythons standard json module, and the source of a
lot of confusion and bugs. Removing this quirk moves ultrajson closer to
the expected behavior.

Instead of trying to coerce serialization ultrajson will now throw a
TypeError: "repr(obj) is not JSON serializable" exception.

Python 3.8.1 (v3.8.1:1b293b6006, Dec 18 2019, 14:08:53)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
(.startup.py)
(imported collections, datetime, itertools, math, os, pprint, re, sys, time)
>>> import numpy
>>> import ujson
>>> ujson.dumps(numpy.bool_(True))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: � is not JSON serializable
>>>

The "�" bit is another issue: #258.

So let's close this. Thanks again for contributing!