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 dictionary containing the percentage of reads mapped to rRNA and MT genes for the given file. If the input files do not exist, the function should raise an appropriate error message.
  
  def calculate_rrna_mt_percent(htseq_counts_file, gtf_file):
      # Implementation details here
      pass
  
  assert calculate_rrna_mt_percent("/path/to/valid_file.txt", "/path/to/valid_gtf_file.gtf") == {"rRNA_percent": 15.32, "MT_percent": 3.25}
  assert calculate_rrna_mt_percent("/path/to/non_existent_file.txt", "/path/to/valid_gtf_file.gtf") == "Error: HTseq_counts file does not exist"
  assert calculate_rrna_mt_percent("/path/to/valid_file.txt", "/path/to/non_existent_gtf_file.gtf") == "Error: GTF file does not exist"
output: def calculate_rrna_mt_percent(htseq_counts_file, gtf_file):
      import os
      if not os.path.exists(htseq_counts_file):
          return "Error: HTseq_counts file does not exist"
      if not os.path.exists(gtf_file):
          return "Error: GTF 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.split('\t')
              if fields[2] == 'gene':
                  gene_info = fields[8]
                  if 'gene_biotype "rRNA"' in gene_info:
                      rrna_genes.add(gene_info.split('gene_name "')[1].split('"')[0])
                  elif 'gene_biotype "Mt_tRNA"' in gene_info or 'gene_biotype "Mt_rRNA"' in gene_info:
                      mt_genes.add(gene_info.split('gene_name "')[1].split('"')[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_reads += count
              if gene_name in rrna_genes:
                  rrna_reads += count
              elif gene_name in mt_genes:
                  mt_reads += count
      
      rrna_percent = (rrna_reads / total_reads) * 100
      mt_percent = (mt_reads / total_reads) * 100
      
      return {"rRNA_percent": rrna_percent, "MT_percent": mt_percent}