cache: improve cache clear error message

lib/cache.js

@@ -56,14 +56,20 @@ const clean = async (args) => {
   }
   const cachePath = path.join(npm.cache, '_cacache')
   if (!npm.flatOptions.force) {
-    throw new Error('As of npm@5, the npm cache self-heals from corruption ' +
-      'issues and data extracted from the cache is guaranteed to be valid. ' +
-      'If you want to make sure everything is consistent, use `npm cache ' +
-      'verify` instead. On the other hand, if you\'re debugging an issue ' +
-      'with the installer, you can use `npm install --cache /tmp/empty-cache` ' +
-      'to use a temporary cache instead of nuking the actual one.\n\n' +
-      'If you\'re sure you want to delete the entire cache, rerun this ' +
-      'command with --force.')
+    throw new Error(`As of npm@5, the npm cache self-heals from corruption issues
+by treating integrity mismatches as cache misses.  As a result,
+data extracted from the cache is guaranteed to be valid.  If you
+want to make sure everything is consistent, use \`npm cache verify\`
+instead.  Deleting the cache can only make npm go slower, and is
+not likely to correct any problems you may be encountering!
+
+On the other hand, if you're debugging an issue with the installer,
+or race conditions that depend on the timing of writing to an empty
+cache, you can use \`npm install --cache /tmp/empty-cache\` to use a
+temporary cache instead of nuking the actual one.
+
+If you're sure you want to delete the entire cache, rerun this command
+with --force.`)
   }
   return rimraf(cachePath)
 }