FIX - wcst_import should ignore failed evaluated metadata expression

[Bang Pham Huu]

For example, if a netCDF band has this metadata

int dw(scenario, model, year, lon, lat) ;
dw:long_name = "yearly_deep_winter_days_-30C" ;
dw:units = "d" ;
dw:description = "Number of Deep Winter Days, calculated over a yearly frequency with a daily minimum temperature threshold of -30C using xclim.indices.tn_days_below()." ;

then wcst_import tried to evaluate that expression value of description property and it has error:

master.error.runtime_exception.RuntimeException: Runtime error: The following expression could not be evaluated. Provided Expression: Number of Deep Winter Days, calculated over a yearly frequency with a daily minimum temperature threshold of -30C using xclim.indices.tn_days_below().. Instantiated Expression: Number of Deep Winter Days, calculated over a yearly frequency with a daily minimum temperature threshold of -30C using xclim.indices.tn_days_below().. Reason: the evaluated sentence still contains expression(s) which cannot be evaluted.

this should be ignored because the expression comes from metadata.