`debci html update-package` crashes when given an unexisting package

$ ./bin/debci html update-package doesnotexist
Traceback (most recent call last):
	11: from /home/terceiro/src/debian/ci/debci/bin/debci-html:5:in `<main>'
	10: from /usr/share/rubygems-integration/all/gems/thor-1.0.1/lib/thor/base.rb:485:in `start'
	 9: from /usr/share/rubygems-integration/all/gems/thor-1.0.1/lib/thor.rb:392:in `dispatch'
	 8: from /usr/share/rubygems-integration/all/gems/thor-1.0.1/lib/thor/invocation.rb:127:in `invoke_command'
	 7: from /usr/share/rubygems-integration/all/gems/thor-1.0.1/lib/thor/command.rb:27:in `run'
	 6: from /home/terceiro/src/debian/ci/debci/lib/debci/html/cli.rb:16:in `update_package'
	 5: from /home/terceiro/src/debian/ci/debci/lib/debci/html.rb:53:in `update_package'
	 4: from /home/terceiro/src/debian/ci/debci/lib/debci/html.rb:53:in `each'
	 3: from /home/terceiro/src/debian/ci/debci/lib/debci/html.rb:54:in `block in update_package'
	 2: from /home/terceiro/src/debian/ci/debci/lib/debci/html.rb:54:in `each'
	 1: from /home/terceiro/src/debian/ci/debci/lib/debci/html.rb:56:in `block (2 levels) in update_package'
/home/terceiro/src/debian/ci/debci/lib/debci/html.rb:156:in `history': undefined method `prefix' for nil:NilClass (NoMethodError)

Instead, it should print an error messsage saying that the package does not exist, and not do anything.