Added exception if the context is already taken.
Added exception if the context is already taken.

hronoses/vispy,sbtlaarzc/vispy,RebeccaWPerry/vispy,julienr/vispy,jay3sh/vispy,jdreaver/vispy,jdreaver/vispy,sh4wn/vispy,hronoses/vispy,sh4wn/vispy,michaelaye/vispy,RebeccaWPerry/vispy,dchilds7/Deysha-Star-Formation,julienr/vispy,michaelaye/vispy,srinathv/vispy,hronoses/vispy,michaelaye/vispy,inclement/vispy,Eric89GXL/vispy,jay3sh/vispy,QuLogic/vispy,drufat/vispy,sbtlaarzc/vispy,drufat/vispy,jdreaver/vispy,QuLogic/vispy,inclement/vispy,sbtlaarzc/vispy,Eric89GXL/vispy,sh4wn/vispy,jay3sh/vispy,srinathv/vispy,inclement/vispy,kkuunnddaannkk/vispy,QuLogic/vispy,ghisvail/vispy,drufat/vispy,Eric89GXL/vispy,bollu/vispy,kkuunnddaannkk/vispy,ghisvail/vispy,dchilds7/Deysha-Star-Formation,dchilds7/Deysha-Star-Formation,srinathv/vispy,kkuunnddaannkk/vispy,ghisvail/vispy,bollu/vispy,bollu/vispy,RebeccaWPerry/vispy,julienr/vispy