Yosys verbose printing and error handling.

[rikushoney]

Yosys is verbose and prints to stdout/stderr. We should probably intercept those prints and filter them.

Upon error, Yosys also does a hard exit using _Exit -- preventing us from gracefully exiting.

Yosys has a logging framework in log.cc (log.h).

The two main functions of interest are logv and logv_error_with_prefix.

We can probably add our own stream to log_streams and remove the lines that add stdout/stderr to the logging files.

Then, modify logv and logv_error_with_prefix to call our logger and fault handler, respectively.