community[minor]: import fix (#20995)

Issue: When the third-party package is not installed, whenever we need
to `pip install <package>` the ImportError is raised.
But sometimes, the `ValueError` or `ModuleNotFoundError` is raised. It
is bad for consistency.
Change: replaced the `ValueError` or `ModuleNotFoundError` with
`ImportError` when we raise an error with the `pip install <package>`
message.
Note: Ideally, we replace all `try: import... except... raise ... `with
helper functions like `import_aim` or just use the existing
[langchain_core.utils.utils.guard_import](https://api.python.langchain.com/en/latest/utils/langchain_core.utils.utils.guard_import.html#langchain_core.utils.utils.guard_import)
But it would be much bigger refactoring. @baskaryan Please, advice on
this.

libs/community/langchain_community/graphs/ontotext_graphdb_graph.py

@@ -81,7 +81,7 @@ class OntotextGraphDBGraph:
             import rdflib
             from rdflib.plugins.stores import sparqlstore
         except ImportError:
-            raise ValueError(
+            raise ImportError(
                 "Could not import rdflib python package. "
                 "Please install it with `pip install rdflib`."
             )