taskdata.py: improve handling of depends/rdepends

taskdata.py: improve handling of depends/rdepends

Error handling only caught the cause where a dependency did not have
any colon, but ignored the case where more than one was given. Now
"pn:task:garbage" will raise an error instead of ignoring ":garbage".

The error message had a misplaced line break (?) with the full stop
on the next line. Indenting the explanation with a space might have
been intended and is kept.

split() was called three times instead of just once.

Instead of improving the two instances of the code (one for 'depends',
one for 'rdepends'), the common code is now in a helper function.

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>