Fix launch.script to not exit prematurely

Use "return" instead of "exit" where possible, especially in
function definitions.

Also fixed the exit codes to match the LSB spec for some specific
conditions (fixes gh-3521).

Fixes gh-3199, fixes gh-3535

spring-boot-docs/src/main/asciidoc/deployment.adoc

@@ -432,6 +432,9 @@ the default behavior in a script or on the command line:
  but if it is not a symlink, or you want to explicitly set the app name this can be
  useful.
 
+|`RUN_ARGS`
+|The arguments to pass to the program (the Spring Boot app).
+
 |`JAVA_HOME`
 |The location of the `java` executable is discovered by using the `PATH` by default, but
  you can set it explicitly if there is an executable file at `$JAVA_HOME/bin/java`.