The _execute helper was catching OSError, except it
was expecting the same attributes as ProcessExecutionError,
which is incorrect.
Added a separate catch and unit test to ensure that we
at least properly catch and convert the error instead of
raising an error about an attribute that does not exist.
Change-Id: Id47715a5657478e4d9dd10ea7f360b1ededa27de
Closes-Bug: #1715466