handler: simplify parsing valid location constraint. (#4040)

Separate out validating v/s parsing logic in
isValidLocationConstraint() into parseLocationConstraint()
and isValidLocation()

Additionally also set `X-Amz-Bucket-Region` as part of the
common headers for the clients to fallback on in-case of any
region related errors.

cmd/api-headers.go

@@ -36,6 +36,7 @@ func setCommonHeaders(w http.ResponseWriter) {
 	// Set unique request ID for each reply.
 	w.Header().Set(responseRequestIDKey, mustGetRequestID(UTCNow()))
 	w.Header().Set("Server", globalServerUserAgent)
+	w.Header().Set("X-Amz-Bucket-Region", serverConfig.GetRegion())
 	w.Header().Set("Accept-Ranges", "bytes")
 }