Fix AttributeError on exception with field source

trying to fix below error, which happens if the handler raises an
custom exception with `source` field inside

```
            locations: list[SourceLocation] | None = [
>               source.get_location(pos) for pos in positions
                ^^^^^^^^^^^^^^^^^^^
            ]
E           AttributeError: 'str' object has no attribute 'get_location'

.tox/py312/lib/python3.12/site-packages/graphql/error/graphql_error.py:161: AttributeError
```

Author: kimjh12

src/graphql/error/located_error.py

@@ -6,6 +6,7 @@
 from typing import TYPE_CHECKING, Collection
 
 from ..pyutils import inspect
+from ..language.source import is_source
 from .graphql_error import GraphQLError
 
 if TYPE_CHECKING:
@@ -39,7 +40,9 @@ def located_error(
     except AttributeError:
         message = str(original_error)
     try:
-        source = original_error.source  # type: ignore
+        source = original_error.source
+        if not is_source(source):
+            source = None
     except AttributeError:
         source = None
     try:

tests/execution/test_executor.py

@@ -391,6 +391,7 @@ async def nulls_out_error_subtrees():
               asyncRawError
               asyncReturnError
               asyncReturnErrorWithExtensions
+              asyncRawErrorWithSource
             }
             """
         )
@@ -410,10 +411,16 @@ async def nulls_out_error_subtrees():
                     "asyncRawError": GraphQLField(GraphQLString),
                     "asyncReturnError": GraphQLField(GraphQLString),
                     "asyncReturnErrorWithExtensions": GraphQLField(GraphQLString),
+                    "asyncRawErrorWithSource": GraphQLField(GraphQLString),
                 },
             )
         )
 
+        class CustomException(Exception):
+            def __init__(self, message, source):
+                super().__init__(message)
+                self.source = source
+
         # noinspection PyPep8Naming,PyMethodMayBeStatic
         class Data:
             def syncOk(self, _info):
@@ -448,6 +455,9 @@ async def asyncRawError(self, _info):
             async def asyncReturnError(self, _info):
                 return GraphQLError("Error getting asyncReturnError")
 
+            async def asyncRawErrorWithSource(self, _info):
+                raise CustomException("Error getting asyncRawErrorWithSource", source="source")
+
             async def asyncReturnErrorWithExtensions(self, _info):
                 return GraphQLError(
                     "Error getting asyncReturnErrorWithExtensions",
@@ -470,6 +480,7 @@ async def asyncReturnErrorWithExtensions(self, _info):
                 "asyncRawError": None,
                 "asyncReturnError": None,
                 "asyncReturnErrorWithExtensions": None,
+                "asyncRawErrorWithSource": None,
             },
             [
                 {
@@ -518,6 +529,11 @@ async def asyncReturnErrorWithExtensions(self, _info):
                     "path": ["asyncReturnErrorWithExtensions"],
                     "extensions": {"foo": "bar"},
                 },
+                {
+                    "message": "Error getting asyncRawErrorWithSource",
+                    "locations": [(13, 15)],
+                    "path": ["asyncRawErrorWithSource"],
+                },
             ],
         )