Improve asExprOf cast error formatting (#19195)

The intention of this change is to make it simpler to read the error
message.

List the expected type and actual type before the expression. This is
usually the most important information and simpler to parse as the
expression can get quite long. For the expected type, the actual type
and the expression, we print the value in the same line if its String
representation has only one line. Otherwise we print it in the next line
with an indentation and extra new line at the end.

Before:
```
java.lang.Exception: Expr cast exception: ((a: scala.Int) => ({
  val v: scala.Int = a
  Binding.apply[scala.Unit](())
}: Binding[scala.Unit]))
of type: scala.Function1[scala.Int, scala.Unit]
did not conform to type: scala.Function1[scala.Int, Binding[scala.Unit]]

    at scala.quoted.runtime.impl.QuotesImpl.asExprOf(QuotesImpl.scala:76)
    ...
```

Example from #19191

After:
```
scala.quoted.runtime.impl.ExprCastException:
  Expected type: scala.Function1[scala.Int, scala.Unit]
  Actual type: scala.Function1[scala.Int, Binding[scala.Unit]]
  Expression:
    ((a: scala.Int) => ({
      val v: scala.Int = a
      Binding.apply[scala.Unit](())
    }: Binding[scala.Unit]))

    at scala.quoted.runtime.impl.QuotesImpl.asExprOf(QuotesImpl.scala:...)
    ...
```

Author: nicolasstucki

compiler/src/scala/quoted/runtime/impl/ExprCastException.scala

@@ -0,0 +1,20 @@
+package scala.quoted.runtime.impl
+
+import dotty.tools.dotc.ast.tpd.Tree
+import dotty.tools.dotc.core.Contexts.*
+
+class ExprCastException(msg: String) extends Exception(msg)
+
+
+object ExprCastException:
+  def apply(expectedType: String, actualType: String, exprCode: String): ExprCastException =
+    new ExprCastException(
+      s"""|
+          |  Expected type: ${formatLines(expectedType)}
+          |  Actual type: ${formatLines(actualType)}
+          |  Expression: ${formatLines(exprCode)}
+          |""".stripMargin)
+
+  private def formatLines(str: String): String =
+    if !str.contains("\n") then str
+    else str.linesIterator.mkString("\n    ", "\n    ", "\n")

compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala

@@ -23,6 +23,7 @@ import scala.quoted.runtime.{QuoteUnpickler, QuoteMatching}
 import scala.quoted.runtime.impl.printers.*
 
 import scala.reflect.TypeTest
+import dotty.tools.dotc.core.NameKinds.ExceptionBinderName
 
 object QuotesImpl {
 
@@ -70,11 +71,10 @@ class QuotesImpl private (using val ctx: Context) extends Quotes, QuoteUnpickler
       if self.isExprOf[X] then
         self.asInstanceOf[scala.quoted.Expr[X]]
       else
-        throw Exception(
-          s"""Expr cast exception: ${self.show}
-            |of type: ${reflect.Printer.TypeReprCode.show(reflect.asTerm(self).tpe)}
-            |did not conform to type: ${reflect.Printer.TypeReprCode.show(reflect.TypeRepr.of[X])}
-            |""".stripMargin
+        throw ExprCastException(
+          expectedType = reflect.Printer.TypeReprCode.show(reflect.TypeRepr.of[X]),
+          actualType = reflect.Printer.TypeReprCode.show(reflect.asTerm(self).tpe),
+          exprCode = self.show
         )
     }
   end extension