Printer preflight checks

[sssoleileraaa]

Description

Until we decouple export from the client, the client will handle errors and wait until it knows there is a single printer attached before sending a document over to the Export VM to be printed.

Our design is to show a Preparing to print message in the client while the Export VM does the following:

• boots up (it might have been manually shutdown)
• returns PRINTER_NOT_FOUND if the printer cannot be found and or MULTIPLE_PRINTERS_FOUND if multiple printers are found

Also this issue needs to update this code so that we don't always assume that the first printer result from lpinfo -v is the one we want. Instead it should also return MULTIPLE_PRINTERS_FOUND if there is more than one result.

[redshiftzero]

closed in #48