Add a check to SQLite port DB script to ensure that the sqlite database passed to the script exists before trying to port from it (#15306)

Author: H-Shay

Diff for: changelog.d/15306.bugfix

@@ -0,0 +1,2 @@
+Add a check to [SQLite port_db script](https://matrix-org.github.io/synapse/latest/postgres.html#porting-from-sqlite)
+to ensure that the sqlite database passed to the script exists before trying to port from it.

Diff for: synapse/_scripts/synapse_port_db.py

@@ -18,6 +18,7 @@
 import argparse
 import curses
 import logging
+import os
 import sys
 import time
 import traceback
@@ -1326,6 +1327,13 @@ def main() -> None:
         filename="port-synapse.log" if args.curses else None,
     )
 
+    if not os.path.isfile(args.sqlite_database):
+        sys.stderr.write(
+            "The sqlite database you specified does not exist, please check that you have the"
+            "correct path."
+        )
+        sys.exit(1)
+
     sqlite_config = {
         "name": "sqlite3",
         "args": {