Handle encrypted files
Also fnameescape() if opening multiple files
Handle "swap file found" (E325) exception

The user has been warned and asked about how to proceed already.
Catching this exception removes the additional output afterwards:

  Error detected while processing function <SNR>237_open_buffers:
  line   38:
  E325: ATTENTION

Closes #67.
Delete open buffers before loading a new session

Closes #70.