Make setup.sh return non-zero for all errors (#2599)

When building a customized Docker image containing pwndbg using ubuntu:20.04 as base, the build process does not fail even when setup.sh exits because Ubuntu 20.04 isn't supported anymore.

I think setup.sh should make the Docker image build process fail by emitting non-zero return values.

setup.sh

@@ -161,7 +161,7 @@ if linux; then
                 install_pacman
             else
                 echo "\"$distro\" is not supported and your distro don't have a package manager that we support currently."
-                exit
+                exit 2
             fi
             ;;
     esac
@@ -169,7 +169,7 @@ fi
 
 if ! hash gdb; then
     echo "Could not find gdb in $PATH"
-    exit
+    exit 3
 fi
 
 # Find the Python version used by GDB.
@@ -186,7 +186,7 @@ if [[ -z "$is_supported" ]]; then
     echo "Your system has unsupported python version. Please use older pwndbg release:"
     echo "'git checkout 2024.08.29' - python3.8, python3.9"
     echo "'git checkout 2023.07.17' - python3.6, python3.7"
-    exit
+    exit 4
 fi
 
 # Install Poetry