feat: add s3err QuotaExceeded for posix/scoutfs

When fileystem quota exceeded, the gateway will now return the
error:
S3 error: 403 (QuotaExceeded):
Your request was denied due to quota exceeded.

This will help clients to better detect upload errors due to
quota exceeded.

Fixes #483

s3err/s3err.go

@@ -116,6 +116,7 @@ const (
 	ErrExistingObjectIsDirectory
 	ErrObjectParentIsFile
 	ErrDirectoryObjectContainsData
+	ErrQuotaExceeded
 )
 
 var errorCodeResponse = map[ErrorCode]APIError{
@@ -414,6 +415,11 @@ var errorCodeResponse = map[ErrorCode]APIError{
 		Description:    "Directory object contains data payload.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrQuotaExceeded: {
+		Code:           "QuotaExceeded",
+		Description:    "Your request was denied due to quota exceeded.",
+		HTTPStatusCode: http.StatusForbidden,
+	},
 }
 
 // GetAPIError provides API Error for input API error code.