Add error message - Comments.scala:128

This commit adds the semantic object for the ```definition not found``` error.
It is part of the (https://github.com/lampepfl/dotty/issues/1589)[https://github.com/lampepfl/dotty/issues/1589]

Author: Thiago Pereira

src/dotty/tools/dotc/core/Comments.scala

@@ -9,6 +9,7 @@ import util.Positions._
 import util.CommentParsing._
 import util.Property.Key
 import parsing.Parsers.Parser
+import reporting.diagnostic.messages.ProperDefinitionNotFound
 
 object Comments {
   val ContextDoc = new Key[ContextDocstrings]
@@ -125,7 +126,7 @@ object Comments {
               val newName = (tree.name.show + "$" + codePos + "$doc").toTermName
               untpd.DefDef(newName, tree.tparams, tree.vparamss, tree.tpt, tree.rhs)
             case _ =>
-              ctx.error("proper definition was not found in `@usecase`", codePos)
+              ctx.error(ProperDefinitionNotFound(), codePos)
               tree
           }
         }

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

@@ -500,4 +500,13 @@ object messages {
            |which cannot start with ${Red(illegalToken)}.""".stripMargin
     }
   }
+
+  case class ProperDefinitionNotFound()(implicit ctx: Context) extends Message(18) {
+    val kind = "Definition Not Found"
+    val msg = hl"""|Proper definition was not found in ${"@usecase"}"""
+    val explanation = {
+      hl"""|You should use ${"def"} when overriding the method signature on ${"@usecase"}"""
+    }
+  }
+  
 }