qt: Fix random segfault when closing "Choose data directory" dialog

The `pickDataDirectory()` function was calling `exit(0)` to quit
the application when the user closes the dialog without choosing
a data directory.

This is a bad idea because a background thread is created (to
check free space on the drive of the currently selected datadir).
The thread is not stopped and unwound properly, resulting in a potential
race condition somewhere deep in Qt.

So replace the `exit()` by a boolean return value, and let the
stack unwind normally.

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