Fail cleanly if we cannot open a file to write a snapshot.

runtime/bin/main.cc

Index: runtime/bin/main.cc
diff --git a/runtime/bin/main.cc b/runtime/bin/main.cc
index 89c752b2815a24a3d8195c704feac4f029d67bf3..39314b2819497132d5bbaf37489a7287091932b5 100644
--- a/runtime/bin/main.cc
+++ b/runtime/bin/main.cc
@@ -1060,7 +1060,11 @@ static void WriteSnapshotFile(const char* snapshot_directory,
   }
 
   File* file = File::Open(qualified_filename, File::kWriteTruncate);
-  ASSERT(file != NULL);
+  if (file == NULL) {
+    ErrorExit(kErrorExitCode,
+              "Unable to open file %s for writing snapshot\n",
+              qualified_filename);
+  }
 
   if (write_magic_number) {
     // Write the magic number to indicate file is a script snapshot.
@@ -1069,7 +1073,7 @@ static void WriteSnapshotFile(const char* snapshot_directory,
 
   if (!file->WriteFully(buffer, size)) {
     ErrorExit(kErrorExitCode,
-              "Unable to open file %s for writing snapshot\n",
+              "Unable to write file %s for writing snapshot\n",
               qualified_filename);
   }
   file->Release();