Add missing continue if pandoc is not available

[behrmann]

Without the continue, the run on the next line will call die if pandoc is not available, thus breaking the fallback for subsequent formats.

[DaanDeMeyer]

If pandoc is the only format we'll fail silently if we add this

[behrmann]

Running mkosi --doc-format pandoc documentation still prints the warning and since this will only ever be used interactively, I think this is fine.