fix and test for #<NoMethodError: undefined method `IOError' for #<DeepL::DocumentApi:.... when 'File already exists at output path'

[jtzero]

Test commit fails because of malformed function

Failures:

  1) DeepL::DocumentApi#translate_document raises an IOError when the output file already exists
     Failure/Error:
       expect do
         DeepL.document.translate_document(example_doc_path, output_document_path,
           source_lang, target_lang, File.basename(example_doc_path),
           {})
       end.to raise_error(IOError)

       expected IOError, got #<NoMethodError: undefined method `IOError' for #<DeepL::DocumentApi:0x00000001064b7600 @api=#<DeepL:...work_retries=5, @logger=nil>, @http_client=#<Net::HTTP api.deepl.com:443 open=false>>, @options={}>> with backtrace:
         # ./lib/deepl/document_api.rb:94:in `translate_document'
         # ./spec/integration_tests/document_api_spec.rb:43:in `block (4 levels) in <top (required)>'
         # ./spec/integration_tests/document_api_spec.rb:42:in `block (3 levels) in <top (required)>'
         # ./spec/integration_tests/integration_test_utils.rb:161:in `block (3 levels) in <top (required)>'
         # ./spec/integration_tests/integration_test_utils.rb:159:in `block (2 levels) in <top (required)>'
     # ./spec/integration_tests/document_api_spec.rb:42:in `block (3 levels) in <top (required)>'
     # ./spec/integration_tests/integration_test_utils.rb:161:in `block (3 levels) in <top (required)>'
     # ./spec/integration_tests/integration_test_utils.rb:159:in `block (2 levels) in <top (required)>'

Finished in 0.02217 seconds (files took 0.82862 seconds to load)
1 example, 1 failure

Failed examples:

rspec ./spec/integration_tests/document_api_spec.rb:35 # DeepL::DocumentApi#translate_document raises an IOError when the output file already exists

Test passes in fix commit