balance: Improve ledger validation errors reporting

balance.py

 import click
+import sys
 import tomli as toml
 import petl
 import petlutils
@@ -10,6 +11,7 @@ from os import path
 from orders import list_designated_accounts
 
 from beancount import loader
+from beancount.parser.printer import print_errors
 from beancount.core import amount, data, inventory
 
 
@@ -42,7 +44,9 @@ def main():
     ledger = path.normpath(path.expanduser(conf['ledger']))
 
     entries, errors, options = loader.load_file(ledger)
-    assert not errors
+    if errors:
+        print_errors(errors)
+        sys.exit(1)
 
     accounts = set(list_designated_accounts(entries))
     balances = cumulative_balances_by_year(entries, accounts, unpack=True)