input: Write a function to calculate the percentage of transcripts mapping to rRNA and MT genes from an HTseq_counts file and a GTF file. The function should return a tuple containing the percentage of rRNA mapped reads and the percentage of MT mapped reads. If either file does not exist, the function should return an error message and terminate.
  assert calculate_rRNA_MT_percentage("/path/to/GTF/file", "/path/to/HTseq_counts/file") == (10.5, 2.3)
  assert calculate_rRNA_MT_percentage("/invalid/path", "/path/to/HTseq_counts/file") == "Error->\tGTF File: /invalid/path does not exist"
  assert calculate_rRNA_MT_percentage("/path/to/GTF/file", "/invalid/path") == "Error->\tFile: /invalid/path does not exist"
output: def calculate_rRNA_MT_percentage(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_reads = 0
      rRNA_reads = 0
      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)
                  elif '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:
                  rRNA_reads += count
              elif gene_name in MT_genes:
                  MT_reads += count
      
      rRNA_percentage = (rRNA_reads / total_reads) * 100
      MT_percentage = (MT_reads / total_reads) * 100
      
      return (rRNA_percentage, MT_percentage)