input: Write a Python function to convert an HTML file to a PDF file using xhtml2pdf library.
  assert convert_html_to_pdf('input1.html', 'output1.pdf') == 'PDF created successfully'
  assert convert_html_to_pdf('input2.html', 'output2.pdf') == 'PDF created successfully'
  assert convert_html_to_pdf('input3.html', 'output3.pdf') == 'Error creating PDF'
output: def convert_html_to_pdf(input_html, output_pdf):
      from xhtml2pdf import pisa
      
      try:
          with open(input_html, 'r', encoding='utf-8') as html_file:
              html_content = html_file.read()
          
          with open(output_pdf, 'wb') as pdf_file:
              pisa_status = pisa.CreatePDF(html_content, dest=pdf_file)
          
          if pisa_status.err:
              return 'Error creating PDF'
          else:
              return 'PDF created successfully'
      except Exception as e:
          return 'Error creating PDF'