Switch to threaded python server for tests.
fix for #2219, "provide a better error message when file= not found/accessible"

summary: create a new Exception class for missing PDF's, use it in place of generic

add new MissingPDFException to util.js
handle MissingPDF in api.js
handle MissingPDF in viewer.js, using new missing_file_error message
add new missing_file_error to l10n/en-US/viewer.properties
send MissingPDF from WorkerMessageHandler's loadDocument
send MissingPDF from GetDocRequest handler
Merge pull request #2626 from wfwalker/addmissingpdfexception

fix for #2219, "provide a better error message when file= not found/acce...
Merge pull request #2625 from brendandahl/threaded-server

Switch to threaded python server for tests.