input: Write a function to calculate the percentage of transcripts mapping to rRNA from an HTseq_counts file using a provided GTF file. The function should return a summary output file named "HTseq_counts_file_rRNAmtRNACounts.txt" containing the total number of rRNA and MT transcripts and their respective percentages of the total read count. If any input file does not exist, the function should print an error message and terminate. The function signature is `def calculate_rRNA_percentage(htseq_file, gtf_file)`.
  
  assert calculate_rRNA_percentage("/path/to/HTseq_counts/file", "/path/to/GTF/file") == "HTseq_counts_file_rRNAmtRNACounts.txt"
  assert calculate_rRNA_percentage("nonexistent_file.txt", "/path/to/GTF/file") == "\nError->\tFile: nonexistent_file.txt does not exist\n"
  assert calculate_rRNA_percentage("/path/to/HTseq_counts/file", "nonexistent_file.gtf") == "\nError->\tGTF File: nonexistent_file.gtf does not exist\n"
  assert calculate_rRNA_percentage("/path/to/HTseq_counts/file", "/path/to/GTF/file") == "HTseq_counts_file_rRNAmtRNACounts.txt"
output: 
  def calculate_rRNA_percentage(htseq_file, gtf_file):
      import os
      if not os.path.exists(htseq_file):
          print(f"\nError->\tFile: {htseq_file} does not exist\n")
          return
      if not os.path.exists(gtf_file):
          print(f"\nError->\tGTF File: {gtf_file} does not exist\n")
          return
      
      rRNA_count = 0
      mtRNA_count = 0
      total_count = 0
      
      with open(gtf_file, 'r') as gtf:
          rRNA_ids = set()
          mtRNA_ids = set()
          for line in gtf:
              if line.startswith('#'):
                  continue
              fields = line.strip().split('\t')
              if fields[2] == 'gene':
                  gene_info = fields[8].split(';')
                  gene_id = [x for x in gene_info if 'gene_id' in x][0].split('"')[1]
                  if 'rRNA' in fields[8]:
                      rRNA_ids.add(gene_id)
                  elif 'MT' in fields[8]:
                      mtRNA_ids.add(gene_id)
      
      with open(htseq_file, 'r') as htseq:
          for line in htseq:
              if line.startswith('__'):
                  continue
              gene_id, count = line.strip().split('\t')
              count = int(count)
              total_count += count
              if gene_id in rRNA_ids:
                  rRNA_count += count
              elif gene_id in mtRNA_ids:
                  mtRNA_count += count
      
      rRNA_percentage = (rRNA_count / total_count) * 100 if total_count > 0 else 0
      mtRNA_percentage = (mtRNA_count / total_count) * 100 if total_count > 0 else 0
      
      output_file = f"{htseq_file}_rRNAmtRNACounts.txt"
      with open(output_file, 'w') as out:
          out.write(f"Total rRNA transcripts: {rRNA_count}\n")
          out.write(f"Total MT transcripts: {mtRNA_count}\n")
          out.write(f"rRNA percentage: {rRNA_percentage:.2f}%\n")
          out.write(f"MT percentage: {mtRNA_percentage:.2f}%\n")
      
      return output_file
  