Add Node and C++ exception wrappers

[treeform]

Summary

• translate raising proc errors into JavaScript and C++ exceptions
• free returned string buffers on exception paths
• update generic and Pixie Node/C++ tests to expect native exceptions

Tests

• node test_node.js
• C++ generic test compiled and passed
• python tests/test_python.py
• Nim-C-Nim test passed
• Pixie Node test passed
• Pixie C++ test passed
• Pixie render gold diff passed