Root file should exist or TeXpresso will just give up

[let-def]

When starting the process with texpresso foo.tex, foo.tex should already exist.
This is understandable when used as a standalone viewer, but not ideal when remote controlled from an editor: the buffer might have been created in the editor but not yet saved on disk.

TeXpresso protocol allows the editor to communicate the contents of the buffer without writing to disk, however the file name should at least exist on disk (a simple touch foo.tex is sufficient).

When remote controlled, TeXpresso should not exit and rather behave as if the file existed and was empty (waiting for the editor to communicate the actual contents).