…s const (cherry picked from commit db47a6c)
(cherry picked from commit 9ad342c)
…s for placement_finder collision tests
(it works with gcc and friends but non-standard)
When trying to render the map widget in the viewer, we catch exceptions and print out the message to stderr. The only exceptions that are printed are mapnik::config_error messages. mapnik can throw more errors, so this change makes it catch all std::exceptions.