improve error message when `--base-href` argument does not start with `/` (#142667)

Resolves https://github.com/flutter/flutter/issues/137700.

In particular, see [this comment from the thread](https://github.com/flutter/flutter/issues/137700#issuecomment-1920241979) to see exactly what this PR is addressing.

packages/flutter_tools/lib/src/commands/build_web.dart

diff --git a/packages/flutter_tools/lib/src/commands/build_web.dart b/packages/flutter_tools/lib/src/commands/build_web.dart
index cf3f92a..52e2410 100644
--- a/packages/flutter_tools/lib/src/commands/build_web.dart
+++ b/packages/flutter_tools/lib/src/commands/build_web.dart
@@ -166,7 +166,10 @@
     }
     final String? baseHref = stringArg('base-href');
     if (baseHref != null && !(baseHref.startsWith('/') && baseHref.endsWith('/'))) {
-      throwToolExit('base-href should start and end with /');
+      throwToolExit(
+        'Received a --base-href value of "$baseHref"\n'
+        '--base-href should start and end with /',
+      );
     }
     if (!flutterProject.web.existsSync()) {
       throwToolExit('Missing index.html.');

packages/flutter_tools/test/commands.shard/hermetic/build_web_test.dart

diff --git a/packages/flutter_tools/test/commands.shard/hermetic/build_web_test.dart b/packages/flutter_tools/test/commands.shard/hermetic/build_web_test.dart
index a9a1bc1..073bc90 100644
--- a/packages/flutter_tools/test/commands.shard/hermetic/build_web_test.dart
+++ b/packages/flutter_tools/test/commands.shard/hermetic/build_web_test.dart
@@ -355,6 +355,28 @@
     ProcessManager: () => processManager,
     BuildSystem: () => TestBuildSystem.all(BuildResult(success: true)),
   });
+
+  testUsingContext('Rejects --base-href value that does not start with /', () async {
+    final TestWebBuildCommand buildCommand = TestWebBuildCommand(fileSystem: fileSystem);
+    final CommandRunner<void> runner = createTestCommandRunner(buildCommand);
+
+    await expectLater(
+      runner.run(<String>[
+        'build',
+        'web',
+        '--no-pub',
+        '--base-href=i_dont_start_with_a_forward_slash',
+      ]),
+      throwsToolExit(
+        message: 'Received a --base-href value of "i_dont_start_with_a_forward_slash"\n'
+          '--base-href should start and end with /',
+      ),
+    );
+  }, overrides: <Type, Generator>{
+    Platform: () => fakePlatform,
+    FileSystem: () => fileSystem,
+    ProcessManager: () => processManager,
+  });
 }
 
 void setupFileSystemForEndToEndTest(FileSystem fileSystem) {