Add a warning when RUST_SRC_PATH cannot be determined
Add a warning when RUST_SRC_PATH cannot be determined

Fixes: #160

birkenfeld/racer,BonsaiDen/racer,syhkiller/racer,hotpxl/racer,ifraixedes/racer,ajroetker/racer,asantopietro/racer,bluss/racer,Wilfred/racer,pmarcelll/racer,jaxx/racer,racer-rust/racer,sthapaun/racer,BonsaiDen/racer,syhkiller/racer,phildawes/racer,bluss/racer,ifraixedes/racer,syohex/racer,marvelm/racer,sliz1/racer,drewet/racer,dleslie/racer,MattWis/racer,asantopietro/racer,MattWis/racer,ane/racer,rranelli/racer,ane/racer,rranelli/racer,syohex/racer,drewet/racer,alkama/racer,andelf/racer,kbknapp/racer,ajroetker/racer,sthapaun/racer,kbknapp/racer