Introduce a MillException to transport the error condition

main/api/src/mill/api/MillException.scala

@@ -0,0 +1,7 @@
+package mill.api
+
+/**
+ * This exception is specifically handled in [[mill.runner.MillMain]] and [[mill.runner.MillServerMain]]. You can use it, if you need to exit Mill with a nice error message.
+ * @param msg The error message, to be displayed to the user.
+ */
+class MillException(msg: String) extends Exception(msg)

runner/src/mill/runner/MillMain.scala

@@ -1,18 +1,38 @@
 package mill.runner
-import mill.main.BuildInfo
 
 import java.io.{FileOutputStream, PrintStream}
 import java.util.Locale
 import scala.jdk.CollectionConverters._
 import scala.util.Properties
 import mill.java9rtexport.Export
-import mill.api.{DummyInputStream, internal}
-import mill.api.SystemStreams
+import mill.api.{DummyInputStream, MillException, internal, SystemStreams}
 import mill.bsp.{BspContext, BspServerResult}
+import mill.main.BuildInfo
 import mill.util.PrintLogger
 
+import java.lang.reflect.InvocationTargetException
+import scala.util.control.NonFatal
+
 @internal
 object MillMain {
+
+  def handleMillException[T](
+      err: PrintStream,
+      onError: => T
+  ): PartialFunction[Throwable, (Boolean, T)] = {
+    case e: MillException =>
+      err.println(e.getMessage())
+      (false, onError)
+    case e: InvocationTargetException
+        if e.getCause != null && e.getCause.isInstanceOf[MillException] =>
+      err.println(e.getCause.getMessage())
+      (false, onError)
+    case NonFatal(e) =>
+      err.println("An unexpected error occurred")
+      throw e
+      (false, onError)
+  }
+
   def main(args: Array[String]): Unit = {
     val initialSystemStreams = new SystemStreams(System.out, System.err, System.in)
     // setup streams
@@ -57,6 +77,7 @@ object MillMain {
           userSpecifiedProperties0 = Map(),
           initialSystemProperties = sys.props.toMap
         )
+      catch handleMillException(runnerStreams.err, ())
       finally {
         cleanupStreams.foreach(_.close())
       }

runner/src/mill/runner/MillServerMain.scala

@@ -71,17 +71,18 @@ object MillServerMain extends MillServerMain[RunnerState] {
       userSpecifiedProperties: Map[String, String],
       initialSystemProperties: Map[String, String]
   ): (Boolean, RunnerState) = {
-    MillMain.main0(
-      args = args,
-      stateCache = stateCache,
-      mainInteractive = mainInteractive,
-      streams0 = streams,
-      bspLog = None,
-      env = env,
-      setIdle = setIdle,
-      userSpecifiedProperties0 = userSpecifiedProperties,
-      initialSystemProperties = initialSystemProperties
-    )
+    try MillMain.main0(
+        args = args,
+        stateCache = stateCache,
+        mainInteractive = mainInteractive,
+        streams0 = streams,
+        bspLog = None,
+        env = env,
+        setIdle = setIdle,
+        userSpecifiedProperties0 = userSpecifiedProperties,
+        initialSystemProperties = initialSystemProperties
+      )
+    catch MillMain.handleMillException(streams.err, stateCache)
   }
 }