wic: Print error if a partition specifies an invalid --source

wic: Print error if a partition specifies an invalid --source

If a partition specified in a .wks file specifies a nonexistent
--source, print an error to that effect and exit.  The error text also
points the user to a command listing valid sources, and help on adding
a new source plugin.

(From OE-Core rev: 4dff6945d2606cebc0ab26e877c5d8c7b457e179)

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>