If expanding a variable triggers an exception the caller currently has no
way to supress the error message or otherwise handle the siutation. An
example of where this is a problem is "bitbake -e" showing tracebacks and
errors for variables like SRCPV in OE/Poky.
Secondly in a chained expansion fails, log mesages are recorded for
every step of the expansion, not just the innermost error which is
where the real failure occured.
To fix this we introduce a new exception ExpansionError which callers
can handle as appropriate.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
