Improve log message on failed requirements installation for plugin due to missing expected file

[GGoetzelmann]

• specify expected file name for requirements file in log message
  • reasoning: it wouldn't be a default for a python project to have file with this exact name, so the expected file name should be documented / explained. This change provides at least basic explanation on failure.
• bump log level to warning
  • reasoning: not having the need to install any requirements, and therefore lacking the file completely, is likely a very rare case for plugins, so it should not throw an error, but a warning is warranted.