Misleading "Can't lock build graph file" when build directory doesn't exist.

If you have a build directory set that doesn't exist, you get a misleading error about lock files:

:-1: error: Cannot lock build graph file 'C:/Users/Me/Documents/Builds/build-MyProject-ARM-Debug/qtc_ARM_3839d614-debug/qtc_ARM_3839d614-debug.bg': Already locked by 'QtCreator' (PID 3732).

It's especially confusing because PID 3732 is the only instance of QtCreator running.

Fix:

1. Builds should create the path tree needed automatically.
2. Error message should be something like "Failed to create lock file".