When file- or directory-opening primitives get errors, signal those

When file- or directory-opening primitives get errors, signal those
errors as open-file-error conditions.  In those cases, don't use
another condition as the explanation: create a meaninful error string
from the error's context.

This change has these effects:

* All file/directory-opening primitives now signal the same condition.

* RETRY and USE-VALUE handlers are available whenever that condition
  is signalled; previously these handlers were only sometimes
  available, and there was no reasonable way to provide them when they
  were not.