add SSE-KMS not-implemented error handling (#6234)

This commit adds error handling for SSE-KMS requests to
HEAD, GET, PUT and COPY operations. The server responds
with `not implemented` if a client sends a SSE-KMS
request.

cmd/api-errors.go

@@ -129,6 +129,7 @@ const (
 	ErrMaximumExpires
 	ErrSlowDown
 	ErrInvalidPrefixMarker
+	ErrBadRequest
 	// Add new error codes here.
 
 	// SSE-S3 related API errors
@@ -636,6 +637,11 @@ var errorCodeResponse = map[APIErrorCode]APIError{
 		Description:    "Invalid marker prefix combination",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrBadRequest: {
+		Code:           "BadRequest",
+		Description:    "400 BadRequest",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 
 	// FIXME: Actual XML error response also contains the header which missed in list of signed header parameters.
 	ErrUnsignedHeaders: {