input: Write a function to calculate the percent of transcripts mapping to rRNA given an HTseq_counts file and a GTF file. The function should return the percentage of reads that map to rRNA and MT genes. If either file does not exist, the function should raise an exception.
  
  assert calculate_rRNA_percent("/path/to/GTF/file", "/path/to/HTseq_counts/file") == 12.345
  assert calculate_rRNA_percent("/non/existent/GTF/file", "/path/to/HTseq_counts/file") == None
  assert calculate_rRNA_percent("/path/to/GTF/file", "/non/existent/HTseq_counts/file") == None
output: 
  import os
  
  def calculate_rRNA_percent(gtf_file, htseq_counts_file):
      if not os.path.exists(gtf_file) or not os.path.exists(htseq_counts_file):
          raise FileNotFoundError("One or both of the files do not exist.")
      
      rRNA_genes = set()
      MT_genes = set()
      total_reads = 0
      rRNA_MT_reads = 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_name = gene_info.split('gene_name "')[1].split('"')[0]
                      rRNA_genes.add(gene_name)
                  if 'gene_biotype "Mt_rRNA"' in gene_info or 'gene_biotype "Mt_tRNA"' in gene_info:
                      gene_name = gene_info.split('gene_name "')[1].split('"')[0]
                      MT_genes.add(gene_name)
      
      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_reads += count
              if gene_name in rRNA_genes or gene_name in MT_genes:
                  rRNA_MT_reads += count
      
      if total_reads == 0:
          return 0.0
      
      return (rRNA_MT_reads / total_reads) * 100
  