Instead of a python backtrace, tell the user they need to install PyYAML
if they wish to use the --yaml output options.
Fixes [YOCTO #3768].
(From OE-Core rev: 69caf24112c11609eb351bea09817029bca0ff2e)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>