Print helpful error when metadata has invalid id

Adds a functional test for how we'd like augur export v2 to behave when
the user provides metadata without a valid id and then updates the
try/except block in export_v2 to catch and print the error from
`read_metadata` when it occurs.

While we're at it, this commit also prints the error for missing
metadata file to stderr instead of stdout.

augur/export_v2.py

@@ -999,8 +999,11 @@ def run_v2(args):
                 if "strain" not in metadata_file[strain]:
                     metadata_file[strain]["strain"] = strain
         except FileNotFoundError:
-            print(f"ERROR: meta data file ({args.metadata}) does not exist")
+            print(f"ERROR: meta data file ({args.metadata}) does not exist", file=sys.stderr)
             sys.exit(2)
+        except Exception as error:
+            print(f"ERROR: {error}", file=sys.stderr)
+            sys.exit(1)
     else:
         metadata_file = {}
 

tests/functional/export_v2.t

@@ -97,4 +97,17 @@ In this case, the column is "name" (one of the default columns expected by Augur
   {}
   $ rm -f "$TMP/dataset1.json"
 
+Run export with metadata that uses an invalid id column.
+This should fail with a helpful error message.
+
+  $ ${AUGUR} export v2 \
+  >  --tree export_v2/tree.nwk \
+  >  --metadata export_v2/dataset1_metadata_without_valid_id.tsv \
+  >  --node-data export_v2/div_node-data.json export_v2/location_node-data.json \
+  >  --auspice-config export_v2/auspice_config1.json \
+  >  --maintainers "Nextstrain Team" \
+  >  --output "$TMP/dataset1.json" > /dev/null
+  ERROR: None of the possible id columns (('strain', 'name')) were found in the metadata's columns ('invalid_id', 'div', 'mutation_length')
+  [1]
+
   $ popd > /dev/null

tests/functional/export_v2/dataset1_metadata_without_valid_id.tsv

@@ -0,0 +1,7 @@
+invalid_id	div	mutation_length
+tipA	1	1
+tipB	3	1
+tipC	3	1
+tipD	8	3
+tipE	9	4
+tipF	6	1