Harden 0.5.0 upgrade gate in PluginConfig.ready() #14

Follow-up to #13. Two CodeRabbit findings on netbox_custom_objects_tab/__init__.py that did not block the 2.4.0 release but are worth a small patch.

1. Broaden the exception catch in the upgrade gate (major)

PluginConfig.ready() currently catches only FieldDoesNotExist when probing for the is_polymorphic model field. If netbox-custom-objects is not installed at all (the user removed it from PLUGINS but left netbox-custom-objects-tab enabled), the import line above the try block raises ModuleNotFoundError and the friendly ImproperlyConfigured message never fires — the user sees a less actionable Python import error.

Suggested fix: wrap both the import and the _meta.get_field(...) probe in a single try block, catching ModuleNotFoundError, ImportError, AttributeError, and FieldDoesNotExist. Re-raise the same ImproperlyConfigured with the existing upgrade message.

2. Drop the hardcoded "2.4+" version in the gate's error message (minor)

\"netbox-custom-objects-tab 2.4+ requires netbox-custom-objects>=0.5.0. \"

The literal 2.4+ will drift as the plugin version progresses. Either interpolate __version__ from the module, or drop the version qualifier entirely ("netbox-custom-objects-tab requires netbox-custom-objects>=0.5.0") — same information without the maintenance burden.

Acceptance criteria

• __init__.py::ready() produces a friendly ImproperlyConfigured when netbox-custom-objects is missing or older than 0.5.0
• The error message no longer contains a hardcoded plugin version string
• Existing 0.5.0+ behaviour unchanged
• No new tests required (the gate is exercised by deployment, not unit tests)

Out of scope

• Implementing a version-string parse fallback (the behaviour probe is the documented design choice — see PR Add polymorphic Object/MultiObject field support + release 2.4.0 #13 description)
• Changes to the polymorphic discovery code itself