Fail gracefully when a loaded plugin in the pylintrc does not exists

[Pierre-Sassoulas]

Is your feature request related to a problem? Please describe

When a plugin does not exists there is a crash. For example using pylint 2.7.2 with a 2.8.3 pylintrc containing 'pylint.extensions.confusing_elif':

Traceback (most recent call last):
  File "/home/psassoulas/.local/bin/pylint", line 8, in <module>
    sys.exit(run_pylint())
  File "/home/psassoulas/.local/lib/python3.8/site-packages/pylint/__init__.py", line 24, in run_pylint
    PylintRun(sys.argv[1:])
  File "/home/psassoulas/.local/lib/python3.8/site-packages/pylint/lint/run.py", line 317, in __init__
    linter.load_plugin_modules(plugins)
  File "/home/psassoulas/.local/lib/python3.8/site-packages/pylint/lint/pylinter.py", line 503, in load_plugin_modules
    module = astroid.modutils.load_module_from_name(modname)
  File "/home/psassoulas/.local/lib/python3.8/site-packages/astroid/modutils.py", line 210, in load_module_from_name
    return importlib.import_module(dotted_name)
  File "/usr/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 973, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'pylint.extensions.confusing_elif'

Describe the solution you'd like

It would be better to fail gracefully and run all the other checks.