Fix layout of implicit error messages

There were extraneous whitespaces and prepositions.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -671,13 +671,14 @@ trait Implicits { self: Typer =>
               |
               |    ${arg.show.replace("\n", "\n    ")}
               |
-              |But $tpe.explanation}."""
+              |But ${tpe.explanation}."""
         }
     }
+    def location(preposition: String) = if (where.isEmpty) "" else s" $preposition $where"
     arg.tpe match {
       case ambi: AmbiguousImplicits =>
-        msg(s"ambiguous implicit arguments: ${ambi.explanation} of $where")(
-            s"ambiguous implicit arguments of type ${pt.show} found for $where")
+        msg(s"ambiguous implicit arguments: ${ambi.explanation}${location("of")}")(
+            s"ambiguous implicit arguments of type ${pt.show} found${location("for")}")
       case _ =>
         val userDefined =
           for {
@@ -690,7 +691,7 @@ trait Implicits { self: Typer =>
               pt.typeSymbol.typeParams.map(_.name.unexpandedName.toString),
               pt.argInfos)
           }
-        msg(userDefined.getOrElse(em"no implicit argument of type $pt was found for $where"))()
+        msg(userDefined.getOrElse(em"no implicit argument of type $pt was found${location("for")}"))()
     }
   }