Debugger: Robustify disabling of system pretty printers

There seems to be systems producing non-ASCII error messages when
running this in directories with non-ASCII characters. Since we
won't be able to workaround, silence the error to not disturb
further operation.

Change-Id: I527ce10c062273d2f4928f2a8fab79b704de067b
Reviewed-by: Christian Stenger <christian.stenger@qt.io>