Slightly better error printing

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

@@ -580,21 +580,21 @@ object FileError {
     def message = s"Download error: $message0"
   }
   case class NotFound(file: String) extends FileError {
-    def message = s"$file: not found"
+    def message = s"Not found: $file"
   }
   case class ChecksumNotFound(sumType: String, file: String) extends FileError {
-    def message = s"$file: $sumType checksum not found"
+    def message = s"$sumType checksum not found: $file"
   }
   case class WrongChecksum(sumType: String, got: String, expected: String, file: String, sumFile: String) extends FileError {
-    def message = s"$file: $sumType checksum validation failed"
+    def message = s"$sumType checksum validation failed: $file"
   }
 
   sealed trait Recoverable extends FileError
   case class Locked(file: File) extends Recoverable {
-    def message = s"$file: locked"
+    def message = s"Locked: $file"
   }
   case class ConcurrentDownload(url: String) extends Recoverable {
-    def message = s"$url: concurrent download"
+    def message = s"Concurrent download: $url"
   }
 
 }

cli/src/main/scala-2.11/coursier/cli/Helper.scala

@@ -197,14 +197,14 @@ class Helper(
   }
 
   exitIf(res.errors.nonEmpty) {
-    s"\n${res.errors.size} error(s):\n" +
+    s"\nError:\n" +
     res.errors.map { case (dep, errs) =>
       s"  ${dep.module}:${dep.version}:\n${errs.map("    " + _.replace("\n", "    \n")).mkString("\n")}"
     }.mkString("\n")
   }
 
   exitIf(res.conflicts.nonEmpty) {
-    s"${res.conflicts.size} conflict(s):\n${Print.dependenciesUnknownConfigs(res.conflicts.toVector, projCache)}"
+    s"\nConflict:\n${Print.dependenciesUnknownConfigs(res.conflicts.toVector, projCache)}"
   }
 
   val trDeps = res.minDependencies.toVector
@@ -277,7 +277,7 @@ class Helper(
     logger.foreach(_.stop())
 
     exitIf(errors.nonEmpty) {
-      s"  ${errors.size} error(s):\n" +
+      s"  Error:\n" +
       errors.map { case (artifact, error) =>
         s"${artifact.url}: $error"
       }.mkString("\n")