Keep it as an error.

Author: David Ungar

Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraph.swift

@@ -677,7 +677,9 @@ extension ModuleDependencyGraph {
                                        atomically: true)
       return false
     } catch {
-      info.diagnosticEngine.emit(.warning_could_not_write_dep_graph(to: path, error: error))
+      // It would be OK to just warn here because the build record will be
+      // removed so that bogus priors won't be used next time.
+      info.diagnosticEngine.emit(.error_could_not_write_dep_graph(to: path, error: error))
       return true
     }
   }
@@ -1038,11 +1040,12 @@ fileprivate extension DependencyKey.Designator {
 }
 
 extension Diagnostic.Message {
-  fileprivate static func warning_could_not_write_dep_graph(
-    to path: VirtualPath,
-    error: Error
+  // FIXME: This should be a warning, but I've been asked to make this
+  // an error for now.
+  fileprivate static func error_could_not_write_dep_graph(
+    to path: VirtualPath
   ) -> Diagnostic.Message {
-    .warning("could not write driver dependency graph to \(path), \(error.localizedDescription)")
+    .error("could not write driver dependency graph to \(path), \(error.localizedDescription)")
   }
 }