improve exception handling for placeholder replacement

do not ignore bad placeholders and just return empty string,
this could have bad consequences, e.g. with --prefix '{invalidplaceholder}':
a typo in the placeholder name would cause the prefix to be the empty string.

borg/helpers.py

@@ -69,6 +69,10 @@ class NoManifestError(Error):
     """Repository has no manifest."""
 
 
+class PlaceholderError(Error):
+    """Formatting Error: "{}".format({}): {}({})"""
+
+
 def check_extension_modules():
     from . import platform
     if hashindex.API_VERSION != 2:
@@ -552,18 +556,10 @@ def dir_is_tagged(path, exclude_caches, exclude_if_present):
 
 
 def format_line(format, data):
-    # TODO: Filter out unwanted properties of str.format(), because "format" is user provided.
-
     try:
         return format.format(**data)
-    except (KeyError, ValueError) as e:
-        # this should catch format errors
-        print('Error in lineformat: "{}" - reason "{}"'.format(format, str(e)))
     except Exception as e:
-        # something unexpected, print error and raise exception
-        print('Error in lineformat: "{}" - reason "{}"'.format(format, str(e)))
-        raise
-    return ''
+        raise PlaceholderError(format, data, e.__class__.__name__, str(e))
 
 
 def replace_placeholders(text):

borg/testsuite/helpers.py

@@ -10,7 +10,7 @@ import msgpack
 import msgpack.fallback
 import time
 
-from ..helpers import Location, format_file_size, format_timedelta, format_line, make_path_safe, \
+from ..helpers import Location, format_file_size, format_timedelta, format_line, PlaceholderError, make_path_safe, \
     prune_within, prune_split, get_cache_dir, get_keys_dir, Statistics, is_slow_msgpack, \
     yes, TRUISH, FALSISH, DEFAULTISH, \
     StableDict, int_to_bigint, bigint_to_int, parse_timestamp, CompressionSpec, ChunkerParams, \
@@ -887,5 +887,8 @@ def test_format_line():
 
 
 def test_format_line_erroneous():
-    data = dict(foo='bar baz')
-    assert format_line('{invalid}', data) == ''  # TODO: rather raise exception
+    data = dict()
+    with pytest.raises(PlaceholderError):
+        assert format_line('{invalid}', data)
+    with pytest.raises(PlaceholderError):
+        assert format_line('{}', data)