oauthutil: support backend-specific errorHandler
oauthutil: support backend-specific errorHandler

This allows the backend to pass a errorHandler function to the doConfig
function. The webserver will pass the current request as a parameter to
the function.
The function can then examine all paramters and build the AuthError
struct which contains name, code, description of the error. A link to
the docs can be added to the HelpURL field.
oauthutil then takes care of formatting for the HTML response page. The
error details are also returned as an error in the server.err channel
and will be logged to the commandline.

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