"Cannot find module" message should be more explicit, saying where it's looking

With all the vagaries around module loading, it'd be great to give a more verbose, helpful error message for "Cannot find module 'foo'", saying where the compiler looked for the module. That can help clue the user into (1) how TypeScript works and (2) how to fix their issue.

The message text could be semi-static, maybe something like: IF using commonjs loading AND path isn't relative THEN say "Couldn't find module Foo; searched node_modules directory off directory XXXX and its ancestors and the package.json Typings path YYYY.

Better doc around the module loading algorithm helps, but just putting some of that info in the error message makes it very easily discovered. It also helps clue the user into what isn't the problem (e.g. the message above confirms that using commonjs module loading & found Typings in package.json, but something else seems to be the problem).

Start by reading the issue's description of CommonJS module loading, node_modules ancestor search, and package.json Typings paths. Trace the compiler's module-resolution and diagnostic entry points, then identify existing tests for "Cannot find module" errors. Done means the diagnostic explains where resolution searched without obscuring the original failure.