missing backups: check if the s3 end point is valid

s3 api never return NotFound or ENOENT - https://docs.aws.amazon.com/AmazonS3/latest/API/ErrorResponses.html

Sadly, DO/OVH etc just return NotFound instead of NoSuchKey. And we cannot
distinguish easily if we are talking to some s3 server or some random server.
This is applicable for things like say minio where maybe there is something
apache now just giving out 404 / NotFound.

src/storage/s3.js

@@ -155,6 +155,7 @@ function exists(apiConfig, backupFilePath, callback) {
             };
 
             s3.headObject(params, function (error) {
+                if (!Object.keys(this.httpResponse.headers).some(h => h.startsWith('x-amz'))) return callback(new BoxError(BoxError.EXTERNAL_ERROR, 'not a s3 endpoint'));
                 if (error && S3_NOT_FOUND(error)) return callback(null, false);
                 if (error) return callback(new BoxError(BoxError.EXTERNAL_ERROR, error.message || error.code));