Task/cdd 3084 error page

[luketowell]

Description

This PR includes the following:

• New authentication error page for errors that occur whilst someone is attempting to log in to the non-public side of the dashboard.

Fixes #CDD-3084

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Type of change

Please select the options that are relevant.

• 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)
• Tech debt item (this is focused solely on addressing any relevant technical debt)

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Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests at the right levels to prove my change is effective
• I have added screenshots or screen grabs where appropriate
• I have added docstrings in the correct style (google)

[mattjreynolds]

@luketowell Looks like this could clash with the changes @jeanpierrefouche-ukhsa is making in his PR - so probably worth just having a quick conversation to make sure they're going to work as expected when both get merged in.

[sonarqubecloud]

Quality Gate failed

Failed conditions
18.3% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud