improve error msg when optional packages not available

Is your feature request related to a problem? Please describe.

from monai.bundle import ConfigParser                                                                                          
                                                                                                                               
config = {"training": "$monai.handlers"}                                                                                       
parser = ConfigParser(config=config)                                                                                         
parser.get_parsed_content("training")   

when ignite is not installed, the error is:

Traceback (most recent call last):
  File "test.py", line 5, in <module>
    parser.get_parsed_content("training")
  File "/Users/Documents/MONAI/monai/bundle/config_parser.py", line 237, in get_parsed_content
    return self.ref_resolver.get_resolved_content(id=id, **kwargs)
  File "/Users/Documents/MONAI/monai/bundle/reference_resolver.py", line 182, in get_resolved_content
    return self._resolve_one_item(id=id, **kwargs)
  File "/Users/Documents/MONAI/monai/bundle/reference_resolver.py", line 166, in _resolve_one_item
    item.evaluate(globals={f"{self._vars}": self.resolved_content}) if run_eval else item
  File "/Users/Documents/MONAI/monai/bundle/config_item.py", line 367, in evaluate
    return eval(value[len(self.prefix) :], globals_, locals)
  File "<string>", line 1, in <module>
AttributeError: module 'monai' has no attribute 'handlers'

cc @dongyang0122 @Nic-Ma

[Nic-Ma]

Hi @ericspod ,

I totally agree with it, sounds like a good plan. @wyli Do you have any other concerns? I think let’s keep this ticket open for a while.

Thanks.

[ericspod]

It will take time and maybe best not to upset things for the upcoming release, I’m working on bundles for you right now too. The way that we’re importing everything at once with load_submodules perhaps should go but that will need a good bit of testing to make sure that everything continues to behave, so let’s leave this for next version and live with a bad error message for now?