Fix some more improper exception re-raises in the filesystem

datastore test suite.  Add a new test corpus file, located so
as to test the backward compatibility functionality of
FileSystemSource.  Add a test to the suite which ensures that
this new file is found.

stix2/test/stix2_data/malware/malware--6b616fc1-1505-48e3-8b2c-0d19337bff38.json

@@ -0,0 +1,27 @@
+{
+    "type": "malware",
+    "id": "malware--6b616fc1-1505-48e3-8b2c-0d19337bff38",
+    "created_by_ref": "identity--c78cb6e5-0c4b-4611-8297-d1b8b55e40b5",
+    "created": "2017-05-31T21:32:58.226Z",
+    "modified": "2018-11-16T22:54:20.390Z",
+    "name": "Rover",
+    "description": "Rover is malware suspected of being used for espionage purposes. It was used in 2015 in a targeted email sent to an Indian Ambassador to Afghanistan.[[Citation: Palo Alto Rover]]",
+    "labels": [
+        "version four"
+    ],
+    "external_references": [
+        {
+            "source_name": "mitre-attack",
+            "url": "https://attack.mitre.org/wiki/Software/S0090",
+            "external_id": "S0090"
+        },
+        {
+            "source_name": "Palo Alto Rover",
+            "description": "Ray, V., Hayashi, K. (2016, February 29). New Malware \u2018Rover\u2019 Targets Indian Ambassador to Afghanistan. Retrieved February 29, 2016.",
+            "url": "http://researchcenter.paloaltonetworks.com/2016/02/new-malware-rover-targets-indian-ambassador-to-afghanistan/"
+        }
+    ],
+    "object_marking_refs": [
+        "marking-definition--fa42a846-8d90-4e51-bc29-71d5b4802168"
+    ]
+}

stix2/test/test_datastore_filesystem.py

@@ -118,7 +118,7 @@ def rel_fs_store():
             # errno symbolic value, but not the windows meaning...
             if e.errno in (errno.ENOENT, 3):
                 continue
-            raise e
+            raise
 
 
 def test_filesystem_source_nonexistent_folder():
@@ -158,7 +158,7 @@ def test_filesystem_source_get_object(fs_source):
     mal = fs_source.get("malware--6b616fc1-1505-48e3-8b2c-0d19337bff38")
     assert mal.id == "malware--6b616fc1-1505-48e3-8b2c-0d19337bff38"
     assert mal.name == "Rover"
-    assert mal.modified == datetime.datetime(2018, 11, 1, 23, 24, 48, 457000,
+    assert mal.modified == datetime.datetime(2018, 11, 16, 22, 54, 20, 390000,
                                              pytz.utc)
 
 
@@ -200,6 +200,24 @@ def test_filesytem_source_query_multiple(fs_source):
     assert len(is_1.external_references) == 4
 
 
+def test_filesystem_source_backward_compatible(fs_source):
+    # this specific object is outside an "ID" directory; make sure we can get
+    # it.
+    modified = datetime.datetime(2018, 11, 16, 22, 54, 20, 390000, pytz.utc)
+    results = fs_source.query([
+        Filter("type", "=", "malware"),
+        Filter("id", "=", "malware--6b616fc1-1505-48e3-8b2c-0d19337bff38"),
+        Filter("modified", "=", modified)
+    ])
+
+    assert len(results) == 1
+    result = results[0]
+    assert result.type == "malware"
+    assert result.id == "malware--6b616fc1-1505-48e3-8b2c-0d19337bff38"
+    assert result.modified == modified
+    assert result.labels == ["version four"]
+
+
 def test_filesystem_sink_add_python_stix_object(fs_sink, fs_source):
     # add python stix object
     camp1 = Campaign(name="Hannibal",
@@ -434,7 +452,7 @@ def test_filesystem_store_query_single_filter(fs_store):
 
 def test_filesystem_store_empty_query(fs_store):
     results = fs_store.query()  # returns all
-    assert len(results) == 29
+    assert len(results) == 30
     assert "tool--242f3da3-4425-4d11-8f5c-b842886da966" in [obj.id for obj in results]
     assert "marking-definition--fa42a846-8d90-4e51-bc29-71d5b4802168" in [obj.id for obj in results]
 
@@ -448,7 +466,7 @@ def test_filesystem_store_query_multiple_filters(fs_store):
 
 def test_filesystem_store_query_dont_include_type_folder(fs_store):
     results = fs_store.query(Filter("type", "!=", "tool"))
-    assert len(results) == 27
+    assert len(results) == 28
 
 
 def test_filesystem_store_add(fs_store):