correct error handling

[marc-portier]

an important change (draft 07) is in the way errors should be handled:

extract:

If a template processor encounters an error outside of an expression,
such as a character sequence that does not match the grammar,
then processing of the template SHOULD cease, the URI-reference result SHOULD
contain the expanded part of the template followed by the remainder
unexpanded, and the location and type of error SHOULD be indicated to the
invoking application. If an error is encountered inside an expression, such
as an operator or value modifier that it does not recognize or cannot
support, then the expression SHOULD be copied to the result unexpanded,
processing of the remainder of the template SHOULD continue, and the
location and type of error SHOULD be indicated to the invoking application.
If an error occurs, the result returned might not be a valid URI reference;
it will be an incompletely expanded template string that is only intended
for diagnostic use.

we should comply to this.