input: Write a function named `calculate_rRNA_percentage` that takes two arguments: an HTseq_counts file path (`htseq_file`) and a GTF file path (`gtf_file`). The function should read the provided files, extract identifiers for rRNA and MT genes, count the number of reads matching these identifiers, and calculate the percentage of total reads that map to rRNA and MT genes. The output should be written to a text file with a name derived from the `HTseq_counts` file, appended with '_rRNAmtRNACounts.txt'. If the files do not exist, the function should return an error message and terminate.
  
  assert calculate_rRNA_percentage('/path/to/HTseq_counts/file', '/path/to/GTF/file') == "Summary output file: /path/to/HTseq_counts/file_rRNAmtRNACounts.txt"
  assert calculate_rRNA_percentage('/nonexistent/file', '/path/to/GTF/file') == "Error->\tFile: /nonexistent/file does not exist"
  assert calculate_rRNA_percentage('/path/to/HTseq_counts/file', '/nonexistent/GTF/file') == "Error->\tGTF File: /nonexistent/GTF/file does not exist"
output: def calculate_rRNA_percentage(htseq_file, gtf_file):
      import os
      if not os.path.exists(htseq_file):
          return f"Error->\tFile: {htseq_file} does not exist"
      if not os.path.exists(gtf_file):
          return f"Error->\tGTF File: {gtf_file} does not exist"
      
      rRNA_mt_ids = set()
      with open(gtf_file, 'r') as gtf:
          for line in gtf:
              if line.startswith('#'):
                  continue
              fields = line.split('\t')
              if fields[2] == 'gene':
                  gene_info = fields[8]
                  if 'gene_biotype "rRNA"' in gene_info or 'gene_biotype "Mt_rRNA"' in gene_info:
                      gene_id = gene_info.split('gene_id "')[1].split('";')[0]
                      rRNA_mt_ids.add(gene_id)
      
      total_reads = 0
      rRNA_mt_reads = 0
      with open(htseq_file, 'r') as htseq:
          for line in htseq:
              if line.startswith('__'):
                  continue
              gene_id, count = line.rsplit(maxsplit=1)
              count = int(count)
              total_reads += count
              if gene_id in rRNA_mt_ids:
                  rRNA_mt_reads += count
      
      percentage = (rRNA_mt_reads / total_reads) * 100 if total_reads > 0 else 0
      
      output_file = f"{htseq_file}_rRNAmtRNACounts.txt"
      with open(output_file, 'w') as out:
          out.write(f"Total reads: {total_reads}\n")
          out.write(f"rRNA and MT reads: {rRNA_mt_reads}\n")
          out.write(f"Percentage of total reads mapping to rRNA and MT genes: {percentage:.2f}%\n")
      
      return f"Summary output file: {output_file}"