DEV: Log tracebacks from extension errors in 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.