class Vips::FailOn

How sensitive loaders are to errors, from never stop (very insensitive), to stop on the smallest warning (very sensitive).

Each implies the ones before it, so ‘:error` implies `:truncated`, for example.

• ‘:none` never stop

• ‘:truncated` stop on image truncated, nothing else

• ‘:error` stop on serious error or truncation

• ‘:warning` stop on anything, even warnings