Improve the grammar of the unused import message.

src/google/protobuf/compiler/command_line_interface_unittest.cc

@@ -884,7 +884,7 @@ TEST_F(CommandLineInterfaceTest, ReportsTransitiveMisingImports_LeafFirst) {
   Run("protocol_compiler --test_out=$tmpdir "
       "--proto_path=$tmpdir bar.proto foo.proto");
   ExpectWarningSubstring(
-      "bar.proto:2:1: warning: Import unused.proto but not used.");
+      "bar.proto:2:1: warning: Import unused.proto is unused.");
 }
 
 TEST_F(CommandLineInterfaceTest, ReportsTransitiveMisingImports_LeafLast) {
@@ -905,7 +905,7 @@ TEST_F(CommandLineInterfaceTest, ReportsTransitiveMisingImports_LeafLast) {
   Run("protocol_compiler --test_out=$tmpdir "
       "--proto_path=$tmpdir foo.proto bar.proto");
   ExpectWarningSubstring(
-      "bar.proto:2:1: warning: Import unused.proto but not used.");
+      "bar.proto:2:1: warning: Import unused.proto is unused.");
 }
 
 TEST_F(CommandLineInterfaceTest, CreateDirectory) {

src/google/protobuf/descriptor.cc

@@ -7125,7 +7125,7 @@ void DescriptorBuilder::LogUnusedDependency(const FileDescriptorProto& proto,
              unused_dependency_.begin();
          it != unused_dependency_.end(); ++it) {
       // Log warnings for unused imported files.
-      std::string error_message = "Import " + (*it)->name() + " but not used.";
+      std::string error_message = "Import " + (*it)->name() + " is unused.";
       AddWarning((*it)->name(), proto, DescriptorPool::ErrorCollector::IMPORT,
                  error_message);
     }

src/google/protobuf/descriptor_unittest.cc

@@ -3479,7 +3479,7 @@ TEST(CustomOptions, UnusedImportWarning) {
   EXPECT_EQ(
       "custom_options_import.proto: "
       "google/protobuf/unittest_custom_options.proto: IMPORT: Import "
-      "google/protobuf/unittest_custom_options.proto but not used.\n",
+      "google/protobuf/unittest_custom_options.proto is unused.\n",
       error_collector.warning_text_);
   EXPECT_EQ("", error_collector.text_);
 }
@@ -5795,7 +5795,7 @@ TEST_F(ValidationErrorTest, UnusedImportWarning) {
       "  field { name:\"base\" number:1 label:LABEL_OPTIONAL "
       "type_name:\"Base\" }"
       "}",
-      "forward.proto: bar.proto: IMPORT: Import bar.proto but not used.\n");
+      "forward.proto: bar.proto: IMPORT: Import bar.proto is unused.\n");
 }
 
 namespace {