utils/log: Convert exception to string before passing to Logger

Now that we add an event when logging errors, we need to avoid passing an
exception object directly to the logger. Otherwise we end up adding the
unserializable Exception object to the events list, which causes an error when
trying to write the output.

wa/utils/log.py

@@ -171,7 +171,7 @@ def log_error(e, logger, critical=False):
     if isinstance(e, KeyboardInterrupt):
         log_func('Got CTRL-C. Aborting.')
     elif isinstance(e, WAError) or isinstance(e, DevlibError):
-        log_func(e)
+        log_func(str(e))
     elif isinstance(e, subprocess.CalledProcessError):
         tb = get_traceback()
         log_func(tb)