Parser now warns if a keyword doesn't evaluate instead of aborting
Parser now warns if a keyword doesn't evaluate instead of aborting

This approach helps debug the parser by showing the invalid keyword and
prevents notebook tests from hanging if there are parse failures. The
strict behaviour can be restored by setting Parser.abort_on_eval_failure
to True.

Note that the display tests will still catch any problems in tested
notebooks - the display output will change if particular keywords stop
being evaluated correctly.

basnijholt/holoviews,vascotenner/holoviews,mjabri/holoviews,vascotenner/holoviews,basnijholt/holoviews,basnijholt/holoviews,ioam/holoviews,ioam/holoviews,ioam/holoviews,vascotenner/holoviews,mjabri/holoviews,mjabri/holoviews