lib/bb/data.py: improve output for expansion errors

lib/bb/data.py: improve output for expansion errors

Instead of logging the function/variable separately as a NOTE when
failing to expand, re-raise ExpansionError with more contextual
information. This means that the full details are reported in Hob as
well as actually reporting the original error message in any UI where
we previously did not. For example, we used to get this with tab/space
indentation issues in a python function:

NOTE: Error expanding variable populate_packages
ERROR: Unable to parse /path/to/recipename.bb

Now, we will get this:

ERROR: ExpansionError during parsing /path/to/recipename.bb: Failure
 expanding variable populate_packages: IndentationError: unindent does
 not match any outer indentation level (<string>, line 4)

Fixes [YOCTO #3162].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>