fix(launcher): output error messages when child processes exit with error

Version 0.24.0 introduced a bug where child processes would error without outputting
the error message. Fix. See #902.
fix(locators): Missing information in warning/error messages

Webdriver's built-in locators (such as `by.css()`) appeared as
'undefined' in protractor's messages.

For instance, if a locator matched multiple elements, protractor
would print the following message: 'warning: more than one element
found for locator undefined- you may need to be more specific'.
style(launcher): remove unused variable.
docs(release): API generation should be done after version bump
chore(release): version bump, changelog, and API for 0.24.1