Improve message when spring-boot application is not found

[jbonofre]

With the minho-spring-boot application manager, the user can load several spring-boot application via the ConfigService#Applications. For instance, he can define spring-boot applications via minho.json file:

{
  "applications": [
    {
      "name": "one-app",
      "type": "spring-boot",
      "url": "file:./app/my-spring-boot-app-1.0-SNAPSHOT.jar",
      "properties": {
        "args": "--server.port=8181"
      }
    }
  ]
}

If the minho-spring-boot application manager doesn't find the jar on the provided url, the user get this error:

INFO: Starting Spring Boot module file:./app/my-spring-boot-app-1.0-SNAPSHOT.jar
Exception in thread "main" java.lang.IllegalStateException: Can't start lifecycle service
        at org.apache.karaf.minho.boot.service.LifeCycleService.start(LifeCycleService.java:66)
        at org.apache.karaf.minho.boot.service.ServiceRegistry.lambda$start$2(ServiceRegistry.java:128)
        at java.base/java.util.Optional.ifPresent(Optional.java:178)
        at org.apache.karaf.minho.boot.service.ServiceRegistry.start(ServiceRegistry.java:126)
        at org.apache.karaf.minho.boot.Minho.start(Minho.java:59)
        at org.apache.karaf.minho.boot.Main.main(Main.java:63)
        Suppressed: java.lang.RuntimeException: Can't start Spring Boot module file:./app/my-spring-boot-app-1.0-SNAPSHOT.jar
                at org.apache.karaf.minho.springboot.SpringBootApplicationManagerService.lambda$onRegister$0(SpringBootApplicationManagerService.java:57)
                at java.base/java.lang.Iterable.forEach(Iterable.java:75)
                at org.apache.karaf.minho.springboot.SpringBootApplicationManagerService.lambda$onRegister$1(SpringBootApplicationManagerService.java:53)
                at org.apache.karaf.minho.boot.service.LifeCycleService.lambda$start$0(LifeCycleService.java:69)
                at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
                at org.apache.karaf.minho.boot.service.LifeCycleService.start(LifeCycleService.java:67)
                ... 5 more
        Caused by: java.lang.ClassNotFoundException: org.springframework.boot.loader.JarLauncher
                at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:445)
                at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:587)
                at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520)
                at org.apache.karaf.minho.springboot.SpringBootApplicationManagerService.start(SpringBootApplicationManagerService.java:106)
                at org.apache.karaf.minho.springboot.SpringBootApplicationManagerService.lambda$onRegister$0(SpringBootApplicationManagerService.java:55)
                ... 10 more

It could be confusing as the problem is not really ClassNotFoundException: org.springframework.boot.loader.JarLauncher but actually the application jar file is not found.

The minho-spring-boot application manager should first check if the jar exists on the provided URL and throw a clean message if not.