[devel] Fixed VS warnings/errors in vstudio project, tidy up Try/Catch

The Try/Catch blocks were checking that the context had not changed -
unnecessary and cexcept cannot handle separate (potentially interleaved)
contexts anyway.  Changed the code to make the Catch re-instate the context -
this should help the compiler because the context is the png_store and
minimising the need for the compiler to avoid optimizing it away is good.
2 files changed