This function is a copy of bootstrap._InitFileStorage with the following
differences:
  - check constants.ENABLE_SHARED_FILE_STORAGE and not
    constants.ENABLE_FILE_STORAGE
  - use different local variable names
  - one different error string

Thus:
  - move the constant check outside of the function call
  - change error string so it's clear where the error is
  - call the same function twice

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>