Prepare pleroma_ctl for moving master to stable and relax the error

message

rel/files/bin/pleroma_ctl

@@ -35,11 +35,11 @@ detect_branch() {
 	if [ "$branch" = "develop" ]; then
 		echo "develop"
 	elif [ "$branch" = "" ]; then
-		echo "master"
+		echo "stable"
 	else
 		# Note: branch name in version is of SemVer format and may only contain [0-9a-zA-Z-] symbols —
 		#   if supporting releases for more branches, need to ensure they contain only these symbols.
-		echo "Releases are built only for master and develop branches" >&2
+		echo "Can't detect the branch automatically, please specify it by using the --branch option." >&2
 		exit 1
 	fi
 }