Improve error handling in C-API to be more robust

It's possible that another throw happens in error handling.
One case is invalid utf8 in error messages that json wants
to stringify. We could wire it up to handle arbitrary level
of nested throws, but that could lead to endless loops, so
I opted to only wire it for one additional level.

Author: mgreter

src/sass_context.cpp

@@ -36,7 +36,7 @@ extern "C" {
     type sass_context_take_##option (struct Sass_Context* ctx) \
     { type foo = ctx->option; ctx->option = 0; return foo; }
 
-  static int handle_errors(Sass_Context* c_ctx) {
+  static int handle_error(Sass_Context* c_ctx) {
     try {
      throw;
     }
@@ -194,6 +194,14 @@ extern "C" {
     return c_ctx->error_status;
   }
 
+  // allow one error handler to throw another error
+  // this can happen with invalid utf8 and json lib
+  static int handle_errors(Sass_Context* c_ctx) {
+    try { return handle_error(c_ctx); }
+    catch (...) { return handle_error(c_ctx); }
+    return c_ctx->error_status;
+  }
+
   // generic compilation function (not exported, use file/data compile instead)
   static Sass_Compiler* sass_prepare_context (Sass_Context* c_ctx, Context* cpp_ctx) throw()
   {