Improve error handling of missuses of AbortExpansion

compiler/src/dotty/tools/dotc/transform/Splicer.scala

@@ -63,7 +63,9 @@ object Splicer {
       catch {
         case ex: CompilationUnit.SuspendException =>
           throw ex
-        case ex: scala.quoted.runtime.AbortExpansion if ctx.reporter.hasErrors =>
+        case ex: scala.quoted.runtime.AbortExpansion =>
+          if !ctx.reporter.hasErrors then
+            report.error("An AbortExpansion was thrown without any errors reported while expanding a macro. This macro has a bug.", splicePos)
           // errors have been emitted
           EmptyTree
         case ex: StopInterpretation =>

library/src/scala/quoted/runtime/StopMacroExpansion.scala

@@ -2,4 +2,4 @@ package scala.quoted.runtime
 
 /** Throwable used to stop the expansion of a macro after an error was reported */
 @deprecated("Use AbortExpansion", "3.0.0-RC3")
-class StopMacroExpansion extends Throwable
+class StopMacroExpansion extends AbortExpansion

tests/neg-macros/ill-abort.check

@@ -0,0 +1,6 @@
+
+-- Error: tests/neg-macros/ill-abort/quoted_2.scala:1:15 ---------------------------------------------------------------
+1 |def test = fail() // error
+  |           ^^^^^^
+  |           An AbortExpansion was thrown without any errors reported while expanding a macro. This macro has a bug.
+  | This location contains code that was inlined from quoted_1.scala:3

tests/neg-macros/ill-abort/quoted_1.scala

@@ -0,0 +1,7 @@
+import scala.quoted.*
+
+inline def fail(): Unit = ${ impl }
+
+private def impl(using Quotes) : Expr[Unit] =
+  // should never be done without reporting error before (see docs)
+  throw new scala.quoted.runtime.AbortExpansion

tests/neg-macros/ill-abort/quoted_2.scala

@@ -0,0 +1 @@
+def test = fail() // error