Currently, execute() may raise an exception that contains a *translated*
string that starts with 'Exit code: %(returncode)d...' if the returncode
of a process was not 0. find_child_pids() will then check if the
raised exception contains 'Exit code: 1' (to check if the returncode is
1), but in non-English locales this will fail as the 2 strings are not
encoded the same.
This patch adds a new ProcessExecutionError (which inherits from
RuntimeError, so as to not change all the code that currently depends on
execute() returning RuntimeError) which now accepts a returncode. This
can be changed explicitly without depending on the error message.
Later patches can move ProcessExecutionError to neutron-lib, if this is
needed - this patch intends to write the smallest piece of code that can
be backported.
Closes-Bug: #1638273
Change-Id: I85d3bec13e852918eb13e73c1367c70e1f4d34b1
3c1bf8697b