Improve message when spring-boot application is not found

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:

```json
{
"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.