error handling for blocked directories is annoying

[wbolster]

having direnv-mode activated and switching to a file in a directory with an .envrc that is not approved via direnv allow results in annoying behaviour: a big warning buffer pops up.

the readme has instructions to reduce the severity of that warning buffer, but perhaps it can be skipped altogether by showing an appropriate error message in the echo area instead.

this requires detecting this error condition, propagating that up to the caller, which should ‘unload’ direnv (if coming from another dir), and then showing that error instead of (or in addition to) the usual ‘diff’ status output.