draw_features needs to return an error message to GlyphSet_wiggle_and_block
draw_features needs to return an error message to GlyphSet_wiggle_and_block

Ensembl/ensembl-webcode,Ensembl/ensembl-webcode,Ensembl/ensembl-webcode,Ensembl/ensembl-webcode,Ensembl/ensembl-webcode