DEV: Add support for logging tracebacks from interactiveshellapp

It's often frustrating working on IPython extensions designed for the html notebook (or in general, contexts where stdout of the InteractiveShell is not displayed), because if an extension fails to load due to an IPython error, the only message that appears in the logs is:
```
[IPKernelApp] WARNING | Error in loading extension: <extension_name>
```
**EDIT:** This now just adds an exc_info=True argument to the logger in init_extensions.
Don't you want this on the inner extension handler too? That seems likely to catch more errors in loading extensions.

Also, we should probably get rid of the `self.shell.showtraceback()` calls to avoid displaying the traceback twice in terminal IPython.