ledger_entry: Improve error reporting for amount expressions.

This is going to become more important as amount expressions become more
capable.

import2ledger/hooks/ledger_entry.py

@@ -168,11 +168,17 @@ class AccountSplitter:
     def _build_amounts(self, template_vars):
         amount_vars = {k: v for k, v in template_vars.items() if isinstance(v, decimal.Decimal)}
         amount_vars['Decimal'] = decimal.Decimal
+        try:
             amounts = [
                 (account, self._currency_decimal(eval(amount_expr, amount_vars),
                                                  template_vars['currency']))
                 for account, amount_expr in self.splits
             ]
+        except (ArithmeticError, NameError, TypeError, ValueError) as error:
+            raise errors.UserInputConfigurationError(
+                "{}: {}".format(type(error).__name__, error),
+                "template {!r}".format(self.template_name)
+            ) from error
         self._balance_amounts(amounts, template_vars['amount'])
         self._balance_amounts(amounts, -template_vars['amount'])
         return amounts

tests/data/templates.ini

@@ -53,6 +53,16 @@ template =
  Expenses:Banking Fees  (6 if {amount} > 50 else 3)
  Income:Sales  -{amount}
 
+[NondecimalWord]
+template =
+ Income:Sales  -5
+ Assets:Cash   foo
+
+[NondecimalVariable]
+template =
+ Income:Sales  -5
+ Assets:Cash   {payee}
+
 [Empty]
 template =
 

tests/test_hook_ledger_entry.py

@@ -259,3 +259,12 @@ def test_hook_handles_template_undefined():
     entry_data = template_vars('DD', 1)
     assert not run_hook(entry_data, 'Nonexistent')
 
+def test_string_value_is_user_error():
+    entry_data = template_vars('EE', 1)
+    with pytest.raises(errors.UserInputConfigurationError):
+        run_hook(entry_data, 'NondecimalWord')
+
+def test_string_variable_is_user_error():
+    entry_data = template_vars('FF', 1)
+    with pytest.raises(errors.UserInputConfigurationError):
+        run_hook(entry_data, 'NondecimalVariable')