refactor: Prefer use of RequestError and next() function on middlewares

Added:
- New error types such as `IntegrationNotFoundError`,
  `WorkspaceNotFoundError`, `AccountNotFoundError' and more.

Refactored:
- Refactored most of the middlewares and very little number of helper
  functions to use RequestError
- Deleted unused imports

Changed:
- Some of the error types in middlewares changed to more related error
  types.
- Environment variable of 'VERBOSE_ERROR_OUTPUT' changed to more
  reliable validation method in `config/index.ts` as per @dangtony98
  requested.

backend/src/middleware/requireWorkspaceAuth.ts

@@ -1,4 +1,3 @@
-import * as Sentry from '@sentry/node';
 import { Request, Response, NextFunction } from 'express';
 import { validateMembership } from '../helpers/membership';
 import { UnauthorizedRequestError } from '../utils/errors';