input: Write a function to calculate the percent of transcripts mapping to rRNA based on an HTseq_counts file and a GTF file. The function should accept two parameters: the path to the HTseq_counts file and the path to the GTF file. The function should read the files, extract the identifiers of genes/transcripts mapping to the rRNA and MT genes, count the reads matching these identifiers, and return a summary containing the total number of rRNA and MT transcripts and the percentage of mapped reads. If the provided files do not exist, the function should return an error message. The output should be a string summarizing the results in the format: "HT-SEQ file name: <filename>\nGTF file name: <filename>\nTotal no. of rRNA transcripts: <count>\nPercent rRNA mapped reads: <percentage>%\nTotal no. of MT transcripts: <count>\nPercent MT mapped reads: <percentage>%".
  
  assert calculate_rRNA_mapping("/path/to/HTseq_counts/file", "/path/to/GTF/file") == "HT-SEQ file name: /path/to/HTseq_counts/file\nGTF file name: /path/to/GTF/file\nTotal no. of rRNA transcripts: 100\nPercent rRNA mapped reads: 10.0%\nTotal no. of MT transcripts: 50\nPercent MT mapped reads: 5.0%"
  assert calculate_rRNA_mapping("/nonexistent/HTseq_counts/file", "/path/to/GTF/file") == "Error->\tFile: /nonexistent/HTseq_counts/file does not exist"
  assert calculate_rRNA_mapping("/path/to/HTseq_counts/file", "/nonexistent/GTF/file") == "Error->\tGTF File: /nonexistent/GTF/file does not exist"
output: def calculate_rRNA_mapping(htseq_counts_path, gtf_path):
      import os
      if not os.path.exists(htseq_counts_path):
          return f"Error->\tFile: {htseq_counts_path} does not exist"
      if not os.path.exists(gtf_path):
          return f"Error->\tGTF File: {gtf_path} does not exist"
      
      rRNA_ids = set()
      MT_ids = set()
      total_reads = 0
      rRNA_reads = 0
      MT_reads = 0
      
      with open(gtf_path, 'r') as gtf_file:
          for line in gtf_file:
              if line.startswith('#'):
                  continue
              fields = line.strip().split('\t')
              if len(fields) >= 9:
                  attributes = fields[8]
                  if 'gene_biotype "rRNA"' in attributes:
                      rRNA_ids.add(attributes.split('gene_id "')[1].split('";')[0])
                  if 'gene_biotype "Mt_tRNA"' in attributes or 'gene_biotype "Mt_rRNA"' in attributes:
                      MT_ids.add(attributes.split('gene_id "')[1].split('";')[0])
      
      with open(htseq_counts_path, 'r') as htseq_file:
          for line in htseq_file:
              if line.startswith('__'):
                  continue
              gene_id, count = line.strip().split('\t')
              count = int(count)
              total_reads += count
              if gene_id in rRNA_ids:
                  rRNA_reads += count
              if gene_id in MT_ids:
                  MT_reads += count
      
      rRNA_percent = (rRNA_reads / total_reads) * 100 if total_reads > 0 else 0
      MT_percent = (MT_reads / total_reads) * 100 if total_reads > 0 else 0
      
      return (f"HT-SEQ file name: {htseq_counts_path}\n"
              f"GTF file name: {gtf_path}\n"
              f"Total no. of rRNA transcripts: {len(rRNA_ids)}\n"
              f"Percent rRNA mapped reads: {rRNA_percent:.1f}%\n"
              f"Total no. of MT transcripts: {len(MT_ids)}\n"
              f"Percent MT mapped reads: {MT_percent:.1f}%")