Include descriptive text with the specified parameter value
in error, and always check that sources were loaded to avoid
raw Python exceptions when they are not
Since the underlying issues with the Python bindings are now fixed,
in most cases we can safely default to allowing an algorithm to
run in a background thread!!
So now we make this the default, and require individual algorithms
which are NOT thread safe to declare this. This includes algorithms
which directly manipulate the current project or layers (such as
setting layer styles), alter the selections in layers, or which
rely on 3rd party libraries (for now, SAGA and GRASS algorithms
are marked as not thread safe... TODO - someone more familiar with
these libraries can investigate and remove the flag if appropriate).
Also models are marked as non-thread safe. TODO: only flag an
individual model as thread-unsafe if any of its child algorithms
report this flag.
