Added customization for error pages

As the title describes, added a dedicated template for the error pages.

• 404 error - when the page is not found:

• any other error code - when there is an error when running the simulations:

These modifications will allow any other theme to customise the error pages.

The Bootstrap button colors were modified to consider the CAiMIRA banner color (rgb(42, 52, 66))

Note: updated mypy pin not to consider the latest releases (from https://github.com/python/mypy/releases/tag/v1.1.1)