Warn instead of fail when NoConvergence errors occur in world_to_pixel_values
Warn instead of fail when NoConvergence errors occur in world_to_pixel_values

Closes #11446

astropy/astropy,StuartLittlefair/astropy,saimn/astropy,saimn/astropy,mhvk/astropy,larrybradley/astropy,pllim/astropy,astropy/astropy,larrybradley/astropy,larrybradley/astropy,astropy/astropy,astropy/astropy,mhvk/astropy,StuartLittlefair/astropy,saimn/astropy,StuartLittlefair/astropy,mhvk/astropy,pllim/astropy,pllim/astropy,lpsinger/astropy,saimn/astropy,larrybradley/astropy,lpsinger/astropy,larrybradley/astropy,StuartLittlefair/astropy,pllim/astropy,astropy/astropy,StuartLittlefair/astropy,lpsinger/astropy,pllim/astropy,saimn/astropy,mhvk/astropy,mhvk/astropy,lpsinger/astropy,lpsinger/astropy