bc: restore printing of script name on errors

bc: restore printing of script name on errors

Examples:

On stdin, no file name is available:
	$ echo 'print "' | busybox bc
	bc: string end could not be found

When the same error is in file:
	$ busybox bc FILE
	bc 1.30.0.git
	Copyright (c) 2018 Gavin D. Howard and contributors
	Report bugs at: https://github.com/gavinhoward/bc
	This is free software with ABSOLUTELY NO WARRANTY
	FILE: string end could not be found
	ready for more input
	>>>

Line number printing to be added later...

function                                             old     new   delta
bc_error_fmt                                          38      70     +32
bc_posix_error_fmt                                    60      90     +30
bc_vm_run                                           1900    1919     +19
bc_program_read                                      338     355     +17
bc_lex_file                                           15      12      -3
bc_program_stdin_name                                  8       -      -8
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 4/1 up/down: 98/-11)             Total: 87 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>