Print full startup/initialization error

changelog.d/15569.feature

@@ -0,0 +1 @@
+Print full error and stack-trace of any exception that occurs during startup/initialization.

synapse/app/_base.py

@@ -21,6 +21,7 @@
 import sys
 import traceback
 import warnings
+from textwrap import indent
 from typing import (
     TYPE_CHECKING,
     Any,
@@ -208,12 +209,28 @@ def quit_with_error(error_string: str) -> NoReturn:
     sys.exit(1)
 
 
+# via https://peps.python.org/pep-3134/#enhanced-reporting
+def format_exception_chain(exc: BaseException) -> str:
+    if exc.__cause__:
+        result = format_exception_chain(exc.__cause__)
+        result += "\nThe above exception was the direct cause..."
+    elif exc.__context__:
+        result = format_exception_chain(exc.__context__)
+        result += "\nDuring handling of the above exception, ..."
+
+    return "".join(traceback.format_exception(exc))
+
+
 def handle_startup_exception(e: Exception) -> NoReturn:
     # Exceptions that occur between setting up the logging and forking or starting
     # the reactor are written to the logs, followed by a summary to stderr.
     logger.exception("Exception during startup")
+
+    error_string = format_exception_chain(e)
+    indendeted_error_string = indent(error_string, "    ")
+
     quit_with_error(
-        f"Error during initialisation:\n   {e}\nThere may be more information in the logs."
+        f"Error during initialisation:\n{indendeted_error_string}\nThere may be more information in the logs."
     )