Added error message if landlab can't be imported.
Added error message if landlab can't be imported.

landlab/landlab,SiccarPoint/landlab,cmshobe/landlab,csherwood-usgs/landlab,amandersillinois/landlab,laijingtao/landlab,landlab/landlab,ManuSchmi88/landlab,landlab/landlab,ManuSchmi88/landlab,laijingtao/landlab,RondaStrauch/landlab,Carralex/landlab,cmshobe/landlab,Carralex/landlab,amandersillinois/landlab,SiccarPoint/landlab,ManuSchmi88/landlab,csherwood-usgs/landlab,Carralex/landlab,RondaStrauch/landlab,RondaStrauch/landlab,cmshobe/landlab