refactor the 'CannotOverwriteExistingCassetteException' exception, building a more detailed message

The 'CannotOverwriteExistingCassetteException' exception now takes two kwargs, cassette and failed requests, in order to get the request(s) in the cassettes with the less differences and put those details in the exception message.

vcr/errors.py

@@ -1,5 +1,30 @@
 class CannotOverwriteExistingCassetteException(Exception):
-    pass
+    def __init__(self, *args, **kwargs):
+        message = self._get_message(kwargs["cassette"], kwargs["failed_request"])
+        super(CannotOverwriteExistingCassetteException, self).__init__(message)
+
+    def _get_message(self, cassette, failed_request):
+        """Get the final message related to the exception"""
+        # Get the similar requests in the cassette that
+        # have match the most with the request.
+        best_matches = cassette.find_requests_with_most_matches(failed_request)
+        # Build a comprehensible message to put in the exception.
+        best_matches_msg = ""
+        for best_match in best_matches:
+            request, _, failed_matchers_assertion_msgs = best_match
+            best_matches_msg += "Similar request found : (%r).\n" % request
+            for failed_matcher, assertion_msg in failed_matchers_assertion_msgs:
+                best_matches_msg += "Matcher failed : %s\n" "%s\n" % (
+                    failed_matcher,
+                    assertion_msg,
+                )
+        return (
+            "Can't overwrite existing cassette (%r) in "
+            "your current record mode (%r).\n"
+            "No match for the request (%r) was found.\n"
+            "%s"
+            % (cassette._path, cassette.record_mode, failed_request, best_matches_msg)
+        )
 
 
 class UnhandledHTTPRequestError(KeyError):