Readable error messages for syntax errors

[stuhlmueller]

Currently, it's difficult to tell what is wrong with the syntax. Example:

/Users/stuhlmueller/Projects/webppl/src/transforms/cps.js:167
  switch (node.type) {
              ^
TypeError: Cannot read property 'type' of null
    at cps (/Users/stuhlmueller/Projects/webppl/src/transforms/cps.js:167:15)
    at Array.<anonymous> (/Users/stuhlmueller/Projects/webppl/src/transforms/cps.js:308:14)
    at Array.<anonymous> (/Users/stuhlmueller/Projects/webppl/src/syntax.js:30:27)
    at loop (/Users/stuhlmueller/Projects/webppl/src/syntax.js:50:24)
    at /Users/stuhlmueller/Projects/webppl/src/syntax.js:51:16
    at Array.<anonymous> (/Users/stuhlmueller/Projects/webppl/src/syntax.js:35:16)
    at loop (/Users/stuhlmueller/Projects/webppl/src/syntax.js:50:24)
    at /Users/stuhlmueller/Projects/webppl/src/syntax.js:51:16
    at Array.<anonymous> (/Users/stuhlmueller/Projects/webppl/src/syntax.js:35:16)
    at loop (/Users/stuhlmueller/Projects/webppl/src/syntax.js:50:24)

[stuhlmueller]

This has improved as a side-effect of our use of sweet.js. I'm going to close this for now—if anyone gets obscure syntax errors, feel free to reopen.