Provide a nice error message if the "firmware" file doesn't exist

[gprossliner]

Currently if firmware from the project file doens't exist it shows:

RuntimeError: Command failed: .qpyt\tools\win\Unisoc_Fotatools\dtools pacmerge --id PY_FS_U,PS --id PY_FS_B,PS ./build/firmware/8915DM_cat1_open_EG915UEUABR03A06M08_OCPU_QPY_01.300.01.300_merge.pac .qpyt\temp\app.pac .qpyt\out\image.pac
Stdout: failed to open for read: ./build/firmware/8915DM_cat1_open_EG915UEUABR03A06M08_OCPU_QPY_01.300.01.300_merge.pac
failed to parse pac file

This could be a better error message.