javascript: make Uploader always throw the same kind of errors

A DirectUpload may fail for several reasons, and return many types of
errors (string, xhr response, Error objects, etc).

For convenience, wrap all these errors in a FileUploadError object.

- It makes easier for clients of the Uploader class to handle errors;
- It allows to propagate the error code and failure responsability.

app/javascript/shared/activestorage/file-upload-error.js

@@ -0,0 +1,67 @@
+// Error while reading the file client-side
+export const ERROR_CODE_READ = 'file-upload-read-error';
+// Error while creating the empty blob on the server
+export const ERROR_CODE_CREATE = 'file-upload-create-error';
+// Error while uploading the blob content
+export const ERROR_CODE_STORE = 'file-upload-store-error';
+// Error while attaching the blob to the record
+export const ERROR_CODE_ATTACH = 'file-upload-attach-error';
+
+// Failure on the client side (syntax error, error reading file, etc.)
+export const FAILURE_CLIENT = 'file-upload-failure-client';
+// Failure on the server side (typically non-200 response)
+export const FAILURE_SERVER = 'file-upload-failure-server';
+// Failure during the transfert (request aborted, connection lost, etc)
+export const FAILURE_CONNECTIVITY = 'file-upload-failure-connectivity';
+
+/**
+  Represent an error during a file upload.
+  */
+export default class FileUploadError extends Error {
+  constructor(message, status, code) {
+    super(message);
+    this.name = 'FileUploadError';
+    this.status = status;
+    this.code = code;
+  }
+
+  /**
+    Return the component responsible of the error (client, server or connectivity).
+    See FAILURE_* constants for values.
+    */
+  get failureReason() {
+    let isNetworkError = this.code != ERROR_CODE_READ;
+
+    if (isNetworkError && this.status != 0) {
+      return FAILURE_SERVER;
+    } else if (isNetworkError && this.status == 0) {
+      return FAILURE_CONNECTIVITY;
+    } else {
+      return FAILURE_CLIENT;
+    }
+  }
+}
+
+// Convert an error message returned by DirectUpload to a proper error object.
+//
+// This function has two goals:
+// 1. Remove the file name from the DirectUpload error message
+//   (because the filename confuses Sentry error grouping)
+// 2. Create each kind of error on a different line
+//   (so that Sentry knows they are different kind of errors, from
+//   the line they were created.)
+export function errorFromDirectUploadMessage(message) {
+  let matches = message.match(/ Status: [0-9]{1,3}/);
+  let status = (matches && parseInt(matches[0], 10)) || undefined;
+
+  // prettier-ignore
+  if (message.includes('Error reading')) {
+    return new FileUploadError('Error reading file.', status, ERROR_CODE_READ);
+  } else if (message.includes('Error creating')) {
+    return new FileUploadError('Error creating file.', status, ERROR_CODE_CREATE);
+  } else if (message.includes('Error storing')) {
+    return new FileUploadError('Error storing file.', status, ERROR_CODE_STORE);
+  } else {
+    return new FileUploadError(message, status, undefined);
+  }
+}