Don't throw an exception when a node.js module cannot be resolved

Sorry, my recent node.js require PR changed the behavior when a `require`'d module cannot be found. This PR restores the previous behavior of just returning `infer.ANull` for unresolvable modules and adds a test of this behavior.

After https://github.com/marijnh/tern/pull/189, before this patch:
```
require("module_not_found")
```
yields
```
$ bin/test
Ran 245 tests from 29 files.
All passed.

module.js:340
    throw err;
          ^
Error: Cannot find module 'mod_not_found'
    at Function.Module._resolveFilename (module.js:338:15)
    at resolveModule (/home/sqs/src/tern/plugin/node.js:63:26)
    at /home/sqs/src/tern/plugin/node.js:95:16
    at Object.exports.IsCallee.constraint.addType (/home/sqs/src/tern/lib/infer.js:271:9)
    at withWorklist (/home/sqs/src/tern/lib/infer.js:639:21)
    at Object.extend.propagate (/home/sqs/src/tern/lib/infer.js:88:25)
    at inferExprVisitor.CallExpression (/home/sqs/src/tern/lib/infer.js:1006:16)
    at Object.CallExpression (/home/sqs/src/tern/lib/infer.js:853:7)
    at infer (/home/sqs/src/tern/lib/infer.js:1038:39)
    at Object.walk.make.Expression (/home/sqs/src/tern/lib/infer.js:1043:7)
    at c (/home/sqs/src/tern/node_modules/acorn/util/walk.js:43:37)
```

After this PR, it no longer throws an exception.

(I actually prefer hard failures when a module can't be resolved, but I think I am in the minority here.)