add test case
make 'wp install' imply 'empty dir'
add our own wp_die() handler
tests: separate stdout from stderr

The more precise assertions expose several buggy tests.

Also reverts WP_CLI::error() second parameter to $label, since sending
the first line to STDERR and subsequent ones to STDOUT is confusing.

If you want to have a multiline error message, just insert \n
characters.
fix WP_CLI::error() not exiting
Merge pull request #291 from wp-cli/fix-291

"Error Connecting To Database" HTML output