- Optionally get the error as a single line.
- Support access the error as an allocated string.
- PyC_ExceptionBuffer_Simple was always printing to the `stdout` while
PyC_ExceptionBuffer didn't, now either print to the output.
Without this, callers are unable to do anything with the error string.
This commit renames 'execute' to 'run' because:
- This follows Python's "PyRun" which these functions wrap.
- Execution functions can use either exec/eval modes,
making naming awkward (for future API refactoring).
