Account for error message when one of the hierarchical sheets is missing

[seppestas]

When attempting to open a schematic sheet without all of it's hierarchical sheets being available in the filesystem, KiCad gives an error message stating "Failed to open `.sch'.

Pressing OK on this error message causes KiCad to successfully load the schematic sheet, but the export_schematic function times out because it get's stuck on the error message.

E.g:

An approach similar to ignoring the missing libraries message could be used to account for this.