fix(developer): cleanly handle filling a new osk file from layout

Fixes #10315.

When filling from layout, if the .kvks file did not exist, an obscure
error was generated:

    Error: 2908 Error encountered parsing eo_plus.kvks: TypeError: Cannot read properties of null (reading 'byteLength')

Fixed this by checking for file existence before attempting to process,
which then reports a more understandable error of 'file not found'.

Then, needed to tweak the caller in Keyman Developer IDE to remove the
reference to the .kvks file before attempting to build
(chicken-and-egg).