Fixes#1540Fixes#1538Fixes#1513Fixes#1425
Fixes SigV4 authentication and presigned URL error handling. Adds two sets of errors in the `s3err` package for these authentication mechanisms.
* Adds a check to return a custom "not supported" error when `X-Amz-Security-Token` is present in presigned URLs.
* Adds a check to return a custom "not supported" error when the `AWS4-ECDSA-P256-SHA256` algorithm is used in presigned URLs.