Fix error message for invalid Directory Name in Poll files trigger

[stas-fomenko]

STR:

1. Create flow with trigger SFTP: Poll files
2. Input:

• Directory Name: (Invalid) /www/stas/INVALID_name
• other fields: default

3. Retrieve sample

AR: error thrown 'Error! sftp.realPath: No such file /www/stas/INVALID_name'
AR: error thrown 'Error! sftp.realPath: No such directory: /www/stas/INVALID_name'