I'd like to suggest several improvements on how user can control the output to the build log:
1. When using script step the only way to fail the step is to throw exception. But in this case QB prints out to the log the whole script body and claims that the problem is in script evaluation which is very confusing. I would suggest allow:
 - or allow fail a step by returning some error status and do not print out the trace stack in this case
 - or provide any other means to control what will be printed out to the log in the case when exception is thrown.
2. The log processing escapes all HTML tags printed to the log which does not allow "inject" html formatting to the log. I would consider adding "custom" markers which will tell QB use the text verbatim and not html escape it. It will allow to inject some html formatting into log, for example control colors, fonts, adding links etc.
3. I would like to have better control on log formatting, for example log line prefixes.