input: Write a function to calculate the percent of transcripts mapping to rRNA and MT genes from a given HTseq_counts file and a GTF file. The function should return a summary output file named "HTseq_counts_file_rRNAmtRNACounts.txt" containing the counts and percentages of rRNA and MT transcripts. The function should also handle file existence checks and return an error message if the files do not exist.
  assert rRNA_MT_count('/path/to/GTF/file', '/path/to/HTseq_counts/file')==True  # Assumes the files exist and the output file is successfully created
  assert rRNA_MT_count('/non/existent/path', '/path/to/HTseq_counts/file')=='Error->\tGTF File: /non/existent/path does not exist'
  assert rRNA_MT_count('/path/to/GTF/file', '/non/existent/path')=='Error->\tFile: /non/existent/path does not exist'
output: def rRNA_MT_count(gtf_file, htseq_counts_file):
      import os
      if not os.path.exists(gtf_file):
          return f'Error->\tGTF File: {gtf_file} does not exist'
      if not os.path.exists(htseq_counts_file):
          return f'Error->\tFile: {htseq_counts_file} does not exist'
      
      rRNA_genes = set()
      MT_genes = set()
      total_counts = 0
      rRNA_counts = 0
      MT_counts = 0
      
      with open(gtf_file, 'r') as gtf:
          for line in gtf:
              if line.startswith('#'):
                  continue
              fields = line.strip().split('\t')
              if fields[2] == 'gene':
                  gene_info = fields[8]
                  if 'gene_biotype "rRNA"' in gene_info:
                      gene_id = gene_info.split('gene_id "')[1].split('"')[0]
                      rRNA_genes.add(gene_id)
                  elif 'gene_biotype "Mt_tRNA"' in gene_info or 'gene_biotype "Mt_rRNA"' in gene_info:
                      gene_id = gene_info.split('gene_id "')[1].split('"')[0]
                      MT_genes.add(gene_id)
      
      with open(htseq_counts_file, 'r') as htseq:
          for line in htseq:
              if line.startswith('__'):
                  continue
              gene_id, count = line.strip().split('\t')
              count = int(count)
              total_counts += count
              if gene_id in rRNA_genes:
                  rRNA_counts += count
              elif gene_id in MT_genes:
                  MT_counts += count
      
      rRNA_percent = (rRNA_counts / total_counts) * 100 if total_counts > 0 else 0
      MT_percent = (MT_counts / total_counts) * 100 if total_counts > 0 else 0
      
      output_file = f"{htseq_counts_file}_rRNAmtRNACounts.txt"
      with open(output_file, 'w') as out:
          out.write(f"rRNA Counts: {rRNA_counts}\n")
          out.write(f"rRNA Percentage: {rRNA_percent:.2f}%\n")
          out.write(f"MT Counts: {MT_counts}\n")
          out.write(f"MT Percentage: {MT_percent:.2f}%\n")
      
      return True