Update error msg for Parsers.scala:1738 and 1739

These error messages are for aux ctors needing non-implicit params. I'm testing the error messages with this code

```
class Square(val len: Int) {
  // typical cases:
  //  (1) this() parens are forgotten, (2) argument set as implicit
  def this(implicit width: Double) = this(0)
  def this = { this(4) }
}

class MyList(val s: String) {
  def this
```

Author: ljdelight

src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1742,8 +1742,8 @@ object Parsers {
       if (owner == nme.CONSTRUCTOR && (result.isEmpty || (result.head take 1 exists (_.mods is Implicit)))) {
         in.token match {
           case LBRACKET   => syntaxError("no type parameters allowed here")
-          case EOF        => incompleteInputError("auxiliary constructor needs non-implicit parameter list")
-          case _          => syntaxError("auxiliary constructor needs non-implicit parameter list", start)
+          case EOF        => incompleteInputError(AuxConstructorNeedsNonImplicitParameter())
+          case _          => syntaxError(AuxConstructorNeedsNonImplicitParameter(), start)
         }
       }
       result

src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -678,4 +678,22 @@ object messages {
            |$fixedVarInAlternative""".stripMargin
     }
   }
+
+  case class AuxConstructorNeedsNonImplicitParameter()(implicit ctx:Context) extends Message(24) {
+    val kind = "Syntax"
+    val msg = "auxiliary constructor needs non-implicit parameter list"
+    val explanation =
+      hl"""An auxiliary constructor requires a non-implicit parameter list. Auxiliary
+        |constructors are allowed to call either auxiliary or primary constructors,
+        |where the primary constructor is allowed implicit parameters. A common mistake
+        |is to forget the parenthesis on `this` (eg `def this() = ...`).
+        |
+        |Here's an example class with primary and auxiliary constructors:
+        |
+        |class Person(val first: String, val last: String) {
+        |  def this() = this("John", "Doe")
+        |  def this(last: String) = this("John", last)
+        |}""".stripMargin
+  }
+
 }