Call toString on invalid composes reference

[bryanant]

Error handling on invalid references doesn't currently spit out useful data. It would be really nice to be able to see the actual problem point where a composition is found to be invalid (at the very least the line of the file that causes the issue).

[tivac]

For reference, here's what the REPL shows using the error objects returned from modular-css (via postcss) directly.

CssSyntaxError: modular-css-composition: /main.css:8:15: Invalid composes reference

   6 | /* Selector composition */
   7 | .booga {
>  8 |     composes: foog;
     |               ^
   9 |     
  10 |     background: blue;