Suggest a way to resolve Geo tracking DB not having all the migrations

This will help customers resolve the problem on their own.

Also check for PostgreSQL earlier to fix spec.

lib/gitlab/geo/health_check.rb

@@ -2,6 +2,8 @@ module Gitlab
   module Geo
     class HealthCheck
       def self.perform_checks
+        raise NotImplementedError unless Gitlab::Database.postgresql?
+
         return '' unless Gitlab::Geo.secondary?
         return 'The Geo secondary role is disabled.' unless Gitlab::Geo.secondary_role_enabled?
         return 'The Geo database configuration file is missing.' unless self.geo_database_configured?
@@ -11,7 +13,8 @@ module Gitlab
         migration_version = self.get_migration_version.to_i
 
         if database_version != migration_version
-          "Current Geo database version (#{database_version}) does not match latest migration (#{migration_version})."
+          "Current Geo database version (#{database_version}) does not match latest migration (#{migration_version}).\n"\
+          "You may have to run `gitlab-rake geo:db:migrate` as root on the secondary."
         else
           ''
         end

spec/lib/gitlab/geo/health_check_spec.rb

@@ -10,6 +10,20 @@ describe Gitlab::Geo::HealthCheck do
       skip("Not using PostgreSQL") unless Gitlab::Database.postgresql?
     end
 
+    it 'returns a string if database is not fully migrated' do
+      allow(Gitlab::Geo).to receive(:secondary?) { true }
+      allow(Gitlab::Geo).to receive(:secondary_role_enabled?).and_return(true)
+      allow(described_class).to receive(:geo_database_configured?).and_return(true)
+      allow(described_class).to receive(:database_secondary?).and_return(true)
+      allow(described_class).to receive(:get_database_version).and_return('20170101')
+      allow(described_class).to receive(:get_migration_version).and_return('20170201')
+
+      message = subject.perform_checks
+
+      expect(message).to include('Current Geo database version (20170101) does not match latest migration (20170201)')
+      expect(message).to include('gitlab-rake geo:db:migrate')
+    end
+
     it 'returns an empty string when not running on a secondary node' do
       allow(Gitlab::Geo).to receive(:secondary?) { false }