Make error clearer when assigning non-string to string field

Summary:
Context
==

When working with some "convert this object to a Thrift object" code, a mistake in the source object was sending what should be a string field as an integer. The only error raised was
```
    1) TypeError: bad argument type for built-in operation

      File "thrift.python/types.pyx", line 616, in thrift.python.types.Struct.__init__

      File "thrift.python/types.pyx", line 130, in thrift.python.types.StringTypeInfo.to_internal_data
```

This error is about as helpful as a chocolate teapot on an open fire, to me.

This change catches the TypeError that's raised by the UTF8String call, and reframes it with context that might help an end user understand precisely what has happened. It would be even better if it would indicate which field is deemed a string in the Thrift spec, and is being passed as not-string to the code, but I don't know how to get at that data in this file.

Implementation
==
* Catch and re-raise TypeError with context details
* Add a unit test that validates the re-raise message

Reviewed By: aristidisp

Differential Revision: D53225702

fbshipit-source-id: e926d5af4eabee61f7548460c87843204b963c6e

thrift/lib/python/test/structs.py

@@ -188,6 +188,13 @@ def test_init_with_invalid_field(self) -> None:
             # pyre-ignore[28]: for test
             easy(val=1, an_int=Integers(small=300), name="foo", val_lists=[1, 2, 3, 4])
 
+    def test_init_with_invalid_field_value(self) -> None:
+        with self.assertRaisesRegex(
+            TypeError, "Cannot create internal string data representation"
+        ):
+            # pyre-ignore[6]: name is string, but under test
+            easy(val=1, an_int=Integers(small=300), name=1)
+
     def test_iterate(self) -> None:
         x = Reserved(
             from_="hello",

thrift/lib/python/types.pyx

@@ -127,7 +127,13 @@ cdef class StringTypeInfo:
 
     # validate and convert to format serializer may understand
     def to_internal_data(self, object value not None):
-        return PyUnicode_AsUTF8String(value)
+        try:
+            return PyUnicode_AsUTF8String(value)
+        except TypeError as e:
+            raise TypeError(
+                "Cannot create internal string data representation. "
+                f"Expected type <class 'str'>, got: {type(value)}."
+            ) from e
 
     # convert deserialized data to user format
     def to_python_value(self, object value):