only warn

Author: David Ungar

Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraph.swift

@@ -675,7 +675,7 @@ extension ModuleDependencyGraph {
                                        bytes: data,
                                        atomically: true)
     } catch {
-      info.diagnosticEngine.emit(.error_could_not_write_dep_graph(to: path))
+      info.diagnosticEngine.emit(.warning_could_not_write_dep_graph(to: path, error: error))
     }
   }
 
@@ -1035,10 +1035,11 @@ fileprivate extension DependencyKey.Designator {
 }
 
 extension Diagnostic.Message {
-  fileprivate static func error_could_not_write_dep_graph(
-    to path: VirtualPath
+  fileprivate static func warning_could_not_write_dep_graph(
+    to path: VirtualPath,
+    error: Error
   ) -> Diagnostic.Message {
-    .error("could not write driver dependency graph to \(path)")
+    .warning("could not write driver dependency graph to \(path), \(error.localizedDescription)")
   }
 }