--caption-path should not autocreate directories unless necessary

[derf]

via mail:

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I have two problems with --caption-path:

• It tries to create a directory (with an error) on readonly
  filesystems such as CDs
• It creates a subdirectories everywhere I use my theme just to view
  images that will never have a caption.

It shouldn't be necessary to have a read-only theme and a read-write
theme for this. My request is to make --caption-path not create a
directory unless a caption is to be written.