input: Write a function to calculate the percentage of transcripts mapping to rRNA and MT genes given an HTseq counts file and a GTF file. The function should return a tuple containing the percentage of rRNA and MT reads respectively. If any input file does not exist, the function should raise a FileNotFoundError.
  
  assert calc_rRNA_MT_percentages("/path/to/HTseq_counts/file", "/path/to/GTF/file") == (15.3, 8.2)
  assert calc_rRNA_MT_percentages("/path/non_existent/HTseq_counts/file", "/path/to/GTF/file") == FileNotFoundError
  assert calc_rRNA_MT_percentages("/path/to/HTseq_counts/file", "/path/non_existent/GTF/file") == FileNotFoundError
output: def calc_rRNA_MT_percentages(htseq_counts_file, gtf_file):
      if not os.path.exists(htseq_counts_file):
          raise FileNotFoundError(f"File not found: {htseq_counts_file}")
      if not os.path.exists(gtf_file):
          raise FileNotFoundError(f"File not found: {gtf_file}")
      
      rRNA_genes = set()
      MT_genes = set()
      
      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_name = re.search(r'gene_name "([^"]+)"', gene_info).group(1)
                      rRNA_genes.add(gene_name)
                  elif 'gene_biotype "Mt_tRNA"' in gene_info or 'gene_biotype "Mt_rRNA"' in gene_info:
                      gene_name = re.search(r'gene_name "([^"]+)"', gene_info).group(1)
                      MT_genes.add(gene_name)
      
      rRNA_count = 0
      MT_count = 0
      total_count = 0
      
      with open(htseq_counts_file, 'r') as htseq:
          for line in htseq:
              if line.startswith('__'):
                  continue
              gene_name, count = line.strip().split('\t')
              count = int(count)
              total_count += count
              if gene_name in rRNA_genes:
                  rRNA_count += count
              elif gene_name in MT_genes:
                  MT_count += count
      
      rRNA_percent = (rRNA_count / total_count) * 100
      MT_percent = (MT_count / total_count) * 100
      
      return (rRNA_percent, MT_percent)