Error context and message (#183)

* POC of error context and message

* Move type errors to the `errors.py` module; Change errors interface a bit

* Rename `.as_dict()` to `.dict()`

* Fix `PydanticErrorMixin` constructor

* Rename `exceptions.py` to `error_wrappers.py`

* Do not include nullable `ctx`

* Fix tests

* Added `int_validator`; Added `IntegerError`

* Added `float_validator`; Added `FloatError`

* Get rid of `__mro__` in prior of `exc.code`

* Removed `min_number_size` and `max_number_size` from config (#174)

* Added `NumberMinSizeError` and `NumberMaxSizeError`

* Added `NoneIsNotAllowedError`

* Added `EnumError`

* Added `path_validator`; Added `PathError`

* Added `DictError`

* Added `ListError`

* Added `TupleError`

* Added `SetError`

* Added `datetime` related errors

* Added `bytes` and `str` related errors

* Added `SequenceError`

* Improved code coverage

* Display error context in string representation of validation error

* Redefine error message templates using config

* Review fixes

* Updated changelog

tests/test_errors.py

@@ -0,0 +1,16 @@
+import pytest
+
+from pydantic import PydanticTypeError
+
+
+def test_pydantic_error():
+    class TestError(PydanticTypeError):
+        code = 'test_code'
+        msg_template = 'test message template "{test_ctx}"'
+
+        def __init__(self, *, test_ctx: int) -> None:
+            super().__init__(test_ctx=test_ctx)
+
+    with pytest.raises(TestError) as exc_info:
+        raise TestError(test_ctx='test_value')
+    assert str(exc_info.value) == 'test message template "test_value"'