Restore the exit codes to the v0.42.0 values

[na--]

I messed up the exit codes in #2815, so this PR now restores the old ones. The only thing different from v0.42.0 is the meaning of the 103 exit code. It used to be GenericEngine, i.e. "the test died from an unknown cause", and will now be used for the well-defined ScriptStoppedFromRESTAPI error.

This is the git diff v0.42.0...restore-exit-codes -- errext/exitcodes/codes.go, for easier review:

diff --git a/errext/exitcodes/codes.go b/errext/exitcodes/codes.go
index abcb602f..8efc0578 100644
--- a/errext/exitcodes/codes.go
+++ b/errext/exitcodes/codes.go
@@ -1,4 +1,5 @@
 // Package exitcodes contains the constants representing possible k6 exit error codes.
+//
 //nolint:golint
 package exitcodes
 
@@ -13,7 +14,7 @@ const (
 	SetupTimeout             ExitCode = 100
 	TeardownTimeout          ExitCode = 101
 	GenericTimeout           ExitCode = 102 // TODO: remove?
-	GenericEngine            ExitCode = 103
+	ScriptStoppedFromRESTAPI ExitCode = 103
 	InvalidConfig            ExitCode = 104
 	ExternalAbort            ExitCode = 105
 	CannotStartRESTAPI       ExitCode = 106