Add better handling of undefined entry points.

crates/core/src/error.rs

@@ -18,5 +18,7 @@ pub enum Error {
     #[from]
     TomlSerialize(toml::ser::Error),
 
+    UndefinedEntryPoint,
+
     UnknownPackage,
 }

crates/core/src/workspace.rs

@@ -158,6 +158,10 @@ impl WorkspaceHandler {
                 .entry_point();
         }
 
+        if entry_point.file_name().is_none() {
+            return Err(Error::UndefinedEntryPoint);
+        }
+
         java::runtime::JVMBuilder::new(Self::DIR_TARGET)
             .assertions(assertions)
             .monitor(true)