Debugging without a binary (regression)

When we attempt to debug a process using GDBserver in standard remote mode
without a symbol file on GDB's end, we may run into an issue where GDB cuts
the connection attempt short due to an error. The error is caused by not
being able to open a symbol file, like so:

--

(gdb) set sysroot
(gdb) tar rem :2345
Remote debugging using :2345
/proc/23769/exe: Permission denied.
(gdb) i r
The program has no registers now.
(gdb)

It should've been like this:

(gdb) set sysroot
(gdb) tar rem :2345
Remote debugging using :2345
warning: /tmp/symbol-file: Permission denied.
0xf7ddb2d0 in ?? ()
(gdb) i r
eax            0x0  0
ecx            0x0  0
edx            0x0  0
ebx            0x0  0
esp            0xffffdfa0 0xffffdfa0
ebp            0x0  0x0
esi            0x0  0
edi            0x0  0
eip            0xf7ddb2d0 0xf7ddb2d0
eflags         0x200  [ IF ]
cs             0x33 51
ss             0x2b 43
ds             0x0  0
es             0x0  0
fs             0x0  0
gs             0x0  0
(gdb)

This is caused by a couple of function calls within exec_file_locate_attach
that can potentially throw errors.

The following patch guards both exec_file_attach and symbol_file_add_main to
prevent the errors from disrupting the connection process.

There was also a case where native GDB tripped on this problem, but it was
mostly fixed by bf74e428.

Regression-tested on x86-64/Ubuntu.

gdb/ChangeLog:

2016-04-13  Luis Machado  <lgustavo@codesourcery.com>

	* exec.c (exec_file_locate_attach): Guard a couple functions
	that can throw errors.
	(exception_print_same): New helper function.