Small fix in error messages

cache/src/main/scala/coursier/Cache.scala

@@ -779,7 +779,7 @@ object Cache {
         checksums = checksums,
         logger = logger,
         pool = pool
-      ).leftMap(_.message).map { f =>
+      ).leftMap(_.describe).map { f =>
         // FIXME Catch error here?
         new String(NioFiles.readAllBytes(f.toPath), "UTF-8")
       }

cache/src/main/scala/coursier/FileError.scala

@@ -5,7 +5,9 @@ import java.io.File
 sealed abstract class FileError(
   val `type`: String,
   val message: String
-) extends Product with Serializable
+) extends Product with Serializable {
+  def describe: String = s"${`type`}: $message"
+}
 
 object FileError {