Provide a friendlier error message when unable to access authentication provider

[nicwells]

Fixes BlueBrain/nexus#2970

Description

Display a more user-friendly error message when authentication provider is not accessible

How has this been tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added necessary unit and integration tests.
• I have added screenshots (if applicable), in the comment section.