fix: catch ETL module import errors

README.md

@@ -47,6 +47,12 @@ export DISEASE_NORM_DB_URL="postgresql://postgres@localhost:5432/disease_normali
 
 Use the `disease_norm_update` command in a shell to update the database.
 
+If you encounter an error message like the following, refer to the installation instructions above:
+
+```shell
+"Encountered ModuleNotFoundError attempting to import Mondo. Are ETL dependencies installed?"
+```
+
 #### Update source(s)
 
 The Disease Normalizer currently uses data from the following sources:

src/disease/cli.py

@@ -14,8 +14,6 @@
     DatabaseWriteException,
     create_db,
 )
-from disease.etl import DO, OMIM, Mondo, NCIt, OncoTree  # noqa: F401
-from disease.etl.merge import Merge
 from disease.schemas import SourceName
 
 # Use to lookup class object from source name. Should be one key-value pair
@@ -176,6 +174,13 @@ def _load_source(
     start_load = timer()
 
     # used to get source class name from string
+    try:
+        from disease.etl import DO, OMIM, Mondo, NCIt, OncoTree  # noqa: F401
+    except ModuleNotFoundError as e:
+        click.echo(
+            f"Encountered ModuleNotFoundError attempting to import {e.name}. Are ETL dependencies installed?"
+        )
+        click.get_current_context().exit()
     SourceClass = eval(n.value)  # noqa: N806
 
     source = SourceClass(database=db)
@@ -219,6 +224,15 @@ def _load_merge(db: AbstractDatabase, processed_ids: Set[str]) -> None:
         processed_ids = set()
         for source in SOURCES_FOR_MERGE:
             processed_ids |= db.get_all_concept_ids(source)
+
+    try:
+        from disease.etl.merge import Merge
+    except ModuleNotFoundError as e:
+        click.echo(
+            f"Encountered ModuleNotFoundError attempting to import {e.name}. Are ETL dependencies installed?"
+        )
+        click.get_current_context().exit()
+
     merge = Merge(database=db)
     click.echo("Constructing normalized records...")
     merge.create_merged_concepts(processed_ids)