fix two issues in gdb.ada/mi_catch_ex.exp (re: "exception-message")

fix two issues in gdb.ada/mi_catch_ex.exp (re: "exception-message")

The following patch introduced a new feature related to Ada exception
catchpoints:

    commit e547c119d043f2ecffb70452020ab9150d083a91
    Author: Joel Brobecker <brobecker@adacore.com>
    Date:   Fri Nov 24 17:09:42 2017 -0500
    Subject: (Ada) provide the exception message when hitting an exception catchpoint

Unfortunately, the patch left 2 errors in gdb.ada/mi_catch_ex.exp,
both inside the "continue_to_exception" function:

  1. The exception message on the console can include the exception
     message, and thus this patch adjust the expected output in
     the corresponding gdb_expect call to allow it;
     to allow it.

  2. There was a TCL syntax confusion in "$exception_name(..."
     that caused TCL to evaluate "exception_name as an array,
     rather than as a variable. This patch fixes this by escaping
     the '(' (and the corresponding closing parenthesis, for
     consistency).

gdb/testsuite/ChangeLog:

        * gdb.ada/mi_catch_ex.exp (continue_to_exception): Adjust
        expected output in gdb_expect call to allow the exception
        message to be present as well.  Fix syntax confusion to avoid
        TCL thinking that exception_name is an array.

Tested on x86_64-linux, with:

    DejaGnu version  1.6
    Expect version   5.45
    Tcl version      8.6