misleading source filename in error message

when ./stack.sh encounters error, the output may look like this:
[ERROR] ./stack.sh:698 nova-api did not start

The source filename is wrong. Actually, it should be like this:
[ERROR] /<path_of_devstack>/lib/nova:698 nova-api did not start

stack.sh uses bash as interpreter, which define "Shell Variables"
BASH_SOURCE:
    An array variable whose members are the source filenames where
    the corresponding shell function names  in  the  FUNCNAME  array
    variable  are  defined.   The  shell  function
    ${FUNCNAME[$i]}  is  defined  in  the  file ${BASH_SOURCE[$i]}
    and called from ${BASH_SOURCE[$i+1]}.

The function "err" is called by function "die" ( and "err_if_not_set",
and "err_if_not_set" is not used at all). ${BASH_SOURCE[2]} will
ignore the deepest two call entries, which corresponding to the shell
functions: "err" and "die".

In one sentence, this change will print the source filename where the
function is defined and exits via function "die".

Fixes bug #1207658

Change-Id: I2aa6642c5cf4cfe781afe278b3dec3e7cba277fa
Merge "misleading source filename in error message"