A problem in OsOperations::exec_command and an error message

[dmitry-lipetsk]

We have two failed tests in TestRemoteOperations:

• test_exec_command_failure
• test_makedirs_and_rmdirs_failure

Both tests expect that ExecUtilException::message contains binary data not string.

There is need to:

• research an implementation of RemoteOperations::exec_command and LocalOperations::exec_command
• understand what is wrong in a message of error
• fix tests