[Rails5] Fix TZInfo::InvalidTimezoneIdentifier exception

In Rails 4.2.10 the `ActiveSupport::TimeZone.find_tzinfo(timezone_name)`
method calls `TZInfo::TimezoneProxy.new(timezone_name)` which returns
`timezone_name` if it is invalid.

But in Rails 5.0 the `ActiveSupport::TimeZone.find_tzinfo(timezone_name)`
method now calls the `TZInfo::Timezone.new(timezone_name)` method which
throws the `TZInfo::InvalidTimezoneIdentifier: Invalid identifier`
exception if `timezone_name` is invalid.

This commit adds the rescue block to return timezone name if the
exception is raised.

Also this change fixes the error:

```
  1) Ci::PipelineSchedule validations does not allow invalid cron patters
     Failure/Error: ActiveSupport::TimeZone.find_tzinfo(timezone).name

     TZInfo::InvalidTimezoneIdentifier:
       Invalid identifier
     # ./lib/gitlab/ci/cron_parser.rb:28:in `timezone_name'
     # ./lib/gitlab/ci/cron_parser.rb:9:in `initialize'
     # ./app/validators/cron_validator.rb:6:in `new'
     # ./app/validators/cron_validator.rb:6:in `validate_each'
     # ./spec/models/ci/pipeline_schedule_spec.rb:26:in `block (3 levels) in <top (required)>'
```