fix: add an error message to say when an email is already taken (#947)

When the email is modified in the user settings and it is already taken by someone else, a generic
message saying that something wrong happened, without saying that it is because the email is already
taken by another user. This PR adds this error message for the email.

server/constants/error.ts

@@ -2,6 +2,7 @@ export enum ApiErrorCode {
   InvalidUrl = 'INVALID_URL',
   InvalidCredentials = 'INVALID_CREDENTIALS',
   InvalidAuthToken = 'INVALID_AUTH_TOKEN',
+  InvalidEmail = 'INVALID_EMAIL',
   NotAdmin = 'NOT_ADMIN',
   SyncErrorGroupedFolders = 'SYNC_ERROR_GROUPED_FOLDERS',
   SyncErrorNoLibraries = 'SYNC_ERROR_NO_LIBRARIES',