Fix exception message when label is not set
Fix exception message when label is not set

sylvestrelucia/grav,dimayakovlev/grav,Pumpapa/lucaswalters.nl,Pumpapa/jacinthawalters.nl,undesign/undesign.github.io,Vivalldi/grav,toovy/grav,getgrav/grav,sylvestrelucia/grav,webuxr/iamrs,JamesPan/gravbox,toovy/grav,Pumpapa/chriswalters.nl,userfrosting/learn,ccardinaux/grav-test,joeyhenricks/a-broke-vegan,JamesPan/gravbox,joeyhenricks/a-broke-vegan,JamesPan/gravbox,sylvestrelucia/grav,undesign/undesign.github.io,tomzx/blog.tomrochette.com,lennerd/grav,JamesPan/gravbox,Silwereth/grav,userfrosting/learn,shortsofia/testesitegrav.github.io,tomzx/blog.tomrochette.com,gigago/grav,stell/grav,Vivalldi/grav,tomzx/blog.tomrochette.com,Pumpapa/lucaswalters.nl,userfrosting/learn,dimayakovlev/grav,Sommerregen/grav,notklaatu/grav,sylvestrelucia/grav,tcsizmadia/grav,notklaatu/grav,tomzx/blog.tomrochette.com,getgrav/grav,Pumpapa/lucaswalters.nl,userfrosting/learn