Error implementations (#232)

Solves #216 by adding a check on table open for existence of the
`.lance` file. Does not check for it for remote connections.

python/lancedb/db.py

@@ -252,7 +252,7 @@ class LanceDBConnection:
         if data is not None:
             tbl = LanceTable.create(self, name, data, schema, mode=mode)
         else:
-            tbl = LanceTable(self, name)
+            tbl = LanceTable.open(self, name)
         return tbl
 
     def open_table(self, name: str) -> LanceTable:
@@ -267,7 +267,7 @@ class LanceDBConnection:
         -------
         A LanceTable object representing the table.
         """
-        return LanceTable(self, name)
+        return LanceTable.open(self, name)
 
     def drop_table(self, name: str):
         """Drop a table from the database.

python/lancedb/table.py

@@ -308,6 +308,19 @@ class LanceTable:
         lance.write_dataset(data, tbl._dataset_uri, mode=mode)
         return tbl
 
+    @classmethod
+    def open(cls, db, name):
+        tbl = cls(db, name)
+        if tbl._conn.is_managed_remote:
+            # Not completely sure how to check for remote table existence yet.
+            return tbl
+        if not os.path.exists(tbl._dataset_uri):
+            raise FileNotFoundError(
+                f"Table {name} does not exist. Please first call db.create_table({name}, data)"
+            )
+
+        return tbl
+
     def delete(self, where: str):
         """Delete rows from the table.