Fix race condition in signal handling, and error handling

[mpokryva]

This PR aims to:

• Fix a race condition in signal handling
• Add correct error message handling in case of a termination by signal

Description:

Line 265 causes a race condition when querying the ProcessState (specifically the WaitStatus) in a goroutine, since cmd.Wait() concurrently modifies it. This PR fixes this race condition, and also adds proper error handling when a signal is raised. The current error handling returns a -1 status when the process exits due to a signal:

ExitCode returns the exit code of the exited process, or -1 if the process hasn't exited or was terminated by a signal (https://golang.org/pkg/os/#ProcessState.ExitCode)