rml2pdf: better message if a font is not available.
rml2pdf: better message if a font is not available.

Since 91422704d965268f, specifying a font that is not registered with
pdfmetrics will raise an exception. Now, improve that exception to
help the user understand what has gone wrong.

Note: rather than hiding the fact that some font is missing, the admin
should see this error and try to either fix the report (to use a known
font), or register more fonts with the customfonts.py mechanism.


akretion/openerp-server,akretion/openerp-server,akretion/openerp-server