Improve error message when unable to locate a package

[matthewp]

Currently the logic expects to basically always be able to find a package:

bit-docs-tag-package/package.js

Lines 39 to 40 in f824863

	var pkgPath = path.join(path.dirname(this.src.path), line.replace("@package", "").trim());
	var pkg = fs.readFileSync(pkgPath).toString();

When for some reason the file doesn't exist it will throw on the fs.readFileSync call and not provide much to help figure out the problem.

We should try/catch this code and provide a better error message.