Remove Abseil dependency to allow cleaner Python bindings

Hello.

I'm working on Python bindings for using this library, and so far I have accomplished this with a Not so clean solution using C++ lambda expressions to avoid issues with the absl::Status returned by the Reset and Update functions of the StrokeModeler class like this:

```cpp
py::class_(handle, "StrokeModeler")
.def(py::init<>())
.def("Reset", [](ink::stroke_model::StrokeModeler &self) -> py::str {
auto status = self.Reset();
if(!status.ok()) {
return py::str(status.message());
}
return py::str("OK");
})
.def("Reset", [](ink::stroke_model::StrokeModeler &self, ink::stroke_model::StrokeModelParams params) -> py::str {
auto status = self.Reset(params);
if(!status.ok()) {
return py::str(status.message());
}
return py::str("OK");
})
.def("Update", [](ink::stroke_model::StrokeModeler &self, const ink::stroke_model::Input& input, std::vector& result) -> py::str {
auto status = self.Update(input, result);
if(!status.ok()) {
return py::str(status.message());
}
return py::str("OK");
})
.def("Save", &ink::stroke_model::StrokeModeler::Save)
.def("Restore", &ink::stroke_model::StrokeModeler::Restore);
```

Here you can see that the Save and Restore bindings are much clear to translate to python.

Another issue is that for this abseil implementation, I need to add into the pybind11 CMakeLists.txt file all dependencies on this project, eg:

```bash
pybind11_add_module(
smooth_lines
wrapper.cpp
#The following dependencies are not needed if there weren't an abseil dependency.
${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/types.cc
${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/params.cc
${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/stroke_modeler.cc

${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/internal/wobble_smoother.cc
${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/internal/stylus_state_modeler.cc

${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/internal/prediction/kalman_predictor.cc
${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/internal/prediction/stroke_end_predictor.cc

${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/internal/prediction/kalman_filter/axis_predictor.cc
${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/internal/prediction/kalman_filter/kalman_filter.cc
)
```

If abseil is not a dependency, the CMakeLists.txt file above would be much cleaner.

My suggestion is to use a custom exception class instead of using Abseil library. Not an expert in C++ so I'm not sure if a custom class is the best option, but removing the Abseil dependency would be great for this use case (and might be the same for other bindings).