mgr/PyModule: fix missing tracebacks in handle_pyerror()

mgr/PyModule: fix missing tracebacks in handle_pyerror()

In certain cases, errors raised in mgr modules don't actually result in a
proper traceback in the mgr log; all you see is a message like "'Hello'
object has no a ttribute 'dneasdfasdf'", but you have no idea where that
came from, which is a complete PITA to debug.

Here's what's going on: handle_pyerror() calls PyErr_Fetch() to get
information about the error that occurred, then passes that information
back to python's traceback.format_exception() function to get the traceback.
If we write code in an mgr module that explicitly raises an exception
(e.g.: 'raise RuntimeError("that didn't work")'), the error value returned
by PyErr_Fetch() is of type RuntimeError, and traceback.format_exception()
does the right thing.  If however we accidentally write code that's just
broken (e.g.: 'self.dneasdfasdf += 1'), the error value returned is not
an actual exception, it's just a string.  So traceback.format_exception()
freaks out with something like "'str' object has no attribute '__cause__'"
(which we don't actually ever see in the logs), which in turn dumps us in a
"catch (error_already_set const &)" block, which just prints out the
single line error string.

https://docs.python.org/3/c-api/exceptions.html#c.PyErr_NormalizeException
tells us that "Under certain circumstances, the values returned by
PyErr_Fetch() below can be “unnormalized”, meaning that *exc is a class
object but *val is not an instance of the same class.".  And that's exactly
the problem we're having here.  We're getting a 'str', not an Exception.
Adding a call to PyErr_NormalizeException() turns the value back into a
proper Exception type and traceback.format_exception() now always does the
right thing.

I've also added calls to peek_pyerror() in the catch blocks, so if anything
else ever somehow causes traceback.format_exception to fail, we'll at least
have an idea of what it is in the log.

Fixes: https://tracker.ceph.com/issues/44799
Signed-off-by: Tim Serong <tserong@suse.com>
(cherry picked from commit dee598087a37623238c35d7595348a4c674c43f3)