Improve error handling if `bwrap` or user namespaces are unavailable

• Do not use exceptions for user-visible error messages

  Throwing an exception does not result in a good error message for users.

• Move bwrap check to the constructor

  This allows buildbox-run-bubblewrap --capabilities to report an error if bwrap is unavailable. BuildStream can forward this error to the user as early as possible.

• Do not incorrectly advertise ISAs if linux32 is not available

• Check whether user namespaces are available

  Fixes #2 (closed).