added ability to set the number of logfile lines displayed on errors (number or 'ALL')

According to:

https://github.com/rock-core/autobuild/issues/6
Integer#to_i is non-validating (i.e. will return I think 0 if the value is not an integer). Use Integer(x) for a validating version, and raise a ConfigError with a proper error message on error so that it is properly shown to the user.