Create a maintenance banner

[Mijago]

Right now, D2AP logs you out when the API is unavailable and does not give any further information.

Goals of this ticket:

• Create an "API unavailable" banner or subpage
• Catch API errors and then show the banner or forward to the subpage

Related issues:

• https://github.com/Mijago/D2ArmorPicker/issues/98

[TheYeshir13]

Should be fixed in 2.6.0