Errors page: add common causes + troubleshooting steps

[maryamsulemani97]

Currently, the errors page has the error message with no further details on how the user can resolve them. We could add two new sections for each error:

• Common causes
• Troubleshooting steps See Plaid, Belvo

See the specifications repository for details on what causes different errors

[guimachiavelli]

I suggest whoever does this also reviews and updates the text. A lot of the older errors have an exceedingly informal tone that does not match the rest of the docs. For example:

[error explanation], check out this guide.

could be rewritten as:

[error message]. For more information, consult the [subject] guide.