bugfix: invalid bit/varbit input causes the log file to be unreadable

The bit/varbit type input functions cause file_fdw to fail to read the
logfile normally.

1. Server conf:
server_encoding = UTF8
locale = zh_CN.UTF-8

2. Create external tables using file_fdw

CREATE EXTENSION file_fdw;
CREATE SERVER pglog FOREIGN DATA WRAPPER file_fdw;

CREATE FOREIGN TABLE pglog (
log_time timestamp(3) with time zone,
user_name text,
database_name text,
process_id integer,
connection_from text,
session_id text,
session_line_num bigint,
command_tag text,
session_start_time timestamp with time zone,
virtual_transaction_id text,
transaction_id bigint,
error_severity text,
sql_state_code text,
message text,
detail text,
hint text,
internal_query text,
internal_query_pos integer,
context text,
query text,
query_pos integer,
location text,
application_name text
) SERVER pglog
OPTIONS ( filename 'log/postgresql-2020-06-16_213409.csv',
format 'csv');

It's normal to be here.

3. bit/varbit input
select b'Ù';

The foreign table cannot be accessed. SELECT * FROM pglog will get:
invalid byte sequence for encoding "UTF8": 0xc3 0x22

The reason is that the error message in the bit_in / varbit_in function
is output directly using %c. Causes the log file to not be decoded
correctly.

The attachment is a patch.