Improve error handling in _build_lookup_table entry points

[ncoop57]

This PR adds error handling when attempting to resolve a particular entry point. When attempting to do this resolution, a particular library might throw an error. For example, this happens with a testcell throws an error due to how it attempts to look into the IPython registration of magics.

entries = {}
for o in pkg_resources.iter_entry_points(group='nbdev'):
    if incl_libs is not None and o.dist.key not in incl_libs: continue
    try: entries[o.name] = _qual_syms(o.resolve())
    except Exception: pass

I've added tests to explicitly mock a good entry point and a bad entry point to test this new functionality:

# Create mock entry points
class BadEntryPoint:
    name = 'bad_entry'
    dist = type('Dist', (), {'key': 'bad_lib'})()
    def resolve(self): raise AttributeError("Simulated error")

class GoodEntryPoint:
    name = 'good_entry'
    dist = type('Dist', (), {'key': 'good_lib'})()
    def resolve(self): return {'syms': {}, 'settings': {}}

from unittest.mock import patch as xpatch
# Clear the cache before testing
_build_lookup_table.cache_clear()

# Patch iter_entry_points
with xpatch('pkg_resources.iter_entry_points', return_value=[BadEntryPoint(), GoodEntryPoint()]):
    entries, py_syms = _build_lookup_table()
    
    # Should only contain the good entry
    assert 'bad_entry' not in entries
    assert 'good_entry' in entries
    assert len(entries) == 1
entries

And all tests are still passing.