Fix `try/catch` in BrowserBase#loadCredentials

The `catch` is intended to only be for JSON parsing errors, but by
wrapping the callback as well it also catches any issues that extend
further down the stack which is undesired behavior–library code should
not be catching errors in application code.
Merge PR #209.
Don't call callback twice on JSON parsing error.
Merge pull request #1 from dropbox/master

Update