1) All slicer's exceptions are now derived from Slic3r::Exception.
2) New exceptions are defined for slicing errors.
3) Exceptions are propagated to the Plater to show.
It remains to modify the slicing back-end to throw the new SlicingError
exceptions instead of std::runtime_error and to show the other exceptions
by a message dialog instead of a notification.
Exceptions thrown by Flow calculation were made explicit classes derived
from std::invalid_argument.
The PresetHints::recommended_thin_wall_thickness() newly catches these
exceptions and it shows the reason of why the hint is invalid.
by the PlaceholderParser.
Namely, all the options with the "ratio_over" reference are now handled
correctly by the PlaceholderParser with the exception
of the "first_layer_extrusion_width", which overrides speed of extrusions
by their respective extrusion type.
Also the various extrusion widths (extrusion_width, first_layer_extrusion_width,
external_perimeter_extrusion_width etc.) produce the same numbers
as if ran through the back-end, with the assumption of not overriding
layer height by the variable layer height editing tool or layer height
modifiers.
