More accurate check for latest version

Summary:
When testing `react-native-git-upgrade` locally I noticed a warning:

    A more recent version of "react-native-git-upgrade" has been found:
    Current: 0.1.0
    Latest: 0.0.1

See https://www.npmjs.com/package/react-native-git-upgrade

This won't happen to a lot of people but better fix the warning. Also, if the check for updates fails, don't crash - the check for updates is not critical to the tool working.

Also, slightly updated one error message.

**Test plan (required)**

Installed `react-native-git-upgrade` locally, ran it inside an app folder (RN 0.29).

Didn't see the wrong "more recent version" warning anymore.

Tried making `checkForUpdates` fail by adding some dummy code: `semver.foo()`. Saw a warning but the process continued:

    git-upgrade WARN Check for latest version failed semver.foo is not a function

Saw a more descriptive error message:

    git-upgrade ERR! Error: react-native version in "package.json" (0.29.0) doesn't match the installed version in "node_mod
Closes https://github.com/facebook/react-native/pull/11188

Differential Revision: D4244002

Pulled By: bestander

fbshipit-source-id: 772044750a933663cb516201d09e2873462cca4a

react-native-git-upgrade/checks.js

@@ -22,8 +22,8 @@ function checkDeclaredVersion(declaredVersion) {
 function checkMatchingVersions(currentVersion, declaredVersion) {
   if (!semver.satisfies(currentVersion, declaredVersion)) {
     throw new Error(
-      'react-native version in "package.json" doesn\'t match ' +
+      'react-native version in "package.json" (' + declaredVersion + ') doesn\'t match ' +
-      'the installed version in "node_modules".\n' +
+      'the installed version in "node_modules" (' + currentVersion + ').\n' +
       'Try running "npm install" to fix this.'
     );
   }

react-native-git-upgrade/cliEntry.js

@@ -156,6 +156,28 @@ function runYeomanGenerators(generatorDir, appName, verbose) {
   return new Promise((resolve) => env.run(generatorArgs, {upgrade: true, force: true}, resolve));
 }
 
+/**
+ * If there's a newer version of react-native-git-upgrade in npm, suggest to the user to upgrade.
+ */
+async function checkForUpdates() {
+  try {
+    log.info('Check for react-native-git-upgrade updates');
+    const lastGitUpgradeVersion = await exec('npm view react-native-git-upgrade@latest version');
+    const current = require('./package').version;
+    const latest = semver.clean(lastGitUpgradeVersion);
+    if (semver.gt(latest, current)) {
+      log.warn(
+        'A more recent version of "react-native-git-upgrade" has been found.\n' +
+        `Current: ${current}\n` +
+        `Latest: ${latest}\n` +
+        'Please run "npm install -g react-native-git-upgrade"'
+      );
+    }
+  } catch (err) {
+    log.warn('Check for latest version failed', err.message);
+  }
+}
+
 async function run(requiredVersion, cliArgs) {
   const context = {
     tmpDir: path.resolve(os.tmpdir(), 'react-native-git-upgrade'),
@@ -165,18 +187,7 @@ async function run(requiredVersion, cliArgs) {
   };
 
   try {
-    log.info('Check for react-native-git-upgrade updates');
+    await checkForUpdates();
-    const lastGitUpgradeVersion = await exec('npm view react-native-git-upgrade@latest version');
-    const current = require('./package').version;
-    const latest = semver.clean(lastGitUpgradeVersion);
-    if (current !== latest) {
-      log.warn(
-        'A more recent version of "react-native-git-upgrade" has been found.\n' +
-        `Current: ${current}\n` +
-        `Latest: ${latest}\n` +
-        'Please run "npm install -g react-native-git-upgrade"'
-      );
-    }
 
     log.info('Read package.json files');
     const {rnPak, pak} = readPackageFiles();