Provide context when resolution fails

When a build failures on browserify, it just complains that it can't find a certain module, it doesn't say where it's trying to find it from.

This doesn't actually identify the source file, but at least tells you what directory you should be searching for problems in.

For example:

```
Error: Cannot find module 'tap' from '/Users/jlfwong/code/node-resolve/example'
```