don't crash when the problematic file isn't there

in practice this happens with generated source files that take
advantage of the //line filename:line syntax to register
alternative file and line number information
See: http://golang.org/pkg/go/token/#File.AddLineInfo

one tool that produces such comments is go yacc
golint found a problem, the //line comments pointed
to my .y file, and the .y file wasn't in the files map
which leads to a panic

this commit isn't perfect, as the error message printed
is somewhat confusing as it references the .y file
but shows code from the generated .go file