storage/stream/flash: Don't update last erased page offset on failure

The stream_flash_erase_page would update stream_flash_ctx member
last_erased_page_start_offset, to page offset it attempted to erase,
even if such operation failed.
The commit changes this behaviour so that in case of failure the
last_erased_page_start_offset would still hold previously, successfully,
erase page offset.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>