Handle std::overflow_error exception from AGG

[jwiggins]

#288 brought in AGG changes from Matplotlib. It also introduced the one and only throw from AGG code. Unfortunately that except is not being caught, so it will result in abort() being called if it is ever triggered.

Related issues:

• Line Plot with 15k points: crash as Qt catches "Exceeded cell block limit" exception chaco#474
• Add exception handling for rasterizer cell limit overflow celiagg/celiagg#96 (fixed in Catch std::overflow_error exceptions from AGG celiagg/celiagg#97)