add custom error message for using crc32c without crt (#2905)

* add custom error message for using crc32c without crt

* pr feedback

botocore/httpchecksum.py

@@ -250,7 +250,14 @@ def resolve_request_checksum_algorithm(
             supported_algorithms = _SUPPORTED_CHECKSUM_ALGORITHMS
 
         algorithm_name = params[algorithm_member].lower()
-        if algorithm_name not in supported_algorithms:
+        if algorithm_name == "crc32c" and not HAS_CRT:
+            raise FlexibleChecksumError(
+                error_msg=(
+                    "Using CRC32C requires an additional dependency. You will "
+                    "need to pip install botocore[crt] before proceeding."
+                )
+            )
+        elif algorithm_name not in supported_algorithms:
             raise FlexibleChecksumError(
                 error_msg="Unsupported checksum algorithm: %s" % algorithm_name
             )

tests/unit/test_httpchecksum.py

@@ -14,6 +14,7 @@
 from io import BytesIO
 
 from botocore.awsrequest import AWSResponse
+from botocore.compat import HAS_CRT
 from botocore.exceptions import AwsChunkedWrapperError, FlexibleChecksumError
 from botocore.httpchecksum import (
     _CHECKSUM_CLS,
@@ -172,6 +173,22 @@ def test_request_checksum_algorithm_model_unsupported_algorithm(self):
                 request, operation_model, params, supported_algorithms=[]
             )
 
+    @unittest.skipIf(HAS_CRT, "Error only expected when CRT is not available")
+    def test_request_checksum_algorithm_model_no_crt_crc32c_unsupported(self):
+        request = self._build_request(b"")
+        operation_model = self._make_operation_model(
+            http_checksum={"requestAlgorithmMember": "Algorithm"},
+        )
+        params = {"Algorithm": "crc32c"}
+        with self.assertRaises(FlexibleChecksumError) as context:
+            resolve_request_checksum_algorithm(
+                request, operation_model, params
+            )
+            self.assertIn(
+                "Using CRC32C requires an additional dependency",
+                str(context.exception),
+            )
+
     def test_request_checksum_algorithm_model_legacy_md5(self):
         request = self._build_request(b"")
         operation_model = self._make_operation_model(required=True)