input: Write a function to calculate the percent of transcripts mapping to rRNA using an HTseq_counts file and a GTF file. The function should return a dictionary containing the total number of rRNA transcripts, the total number of MT transcripts, and their respective percentages of the total read count. If the files do not exist, the function should return an error message.
  Calculate the Percent of Transcripts Mapping to rRNA
  
  assert calculate_rRNA_percent("/path/to/HTseq/counts/file", "/path/to/GTF/file") == {
      "rRNA_transcripts": 12345,
      "MT_transcripts": 678,
      "rRNA_percentage": 15.234,
      "MT_percentage": 0.891
  }
  assert calculate_rRNA_percent("nonexistent_file.txt", "/path/to/GTF/file") == "Error: HTseq_counts file does not exist."
  assert calculate_rRNA_percent("/path/to/HTseq/counts/file", "nonexistent_file.gtf") == "Error: GTF file does not exist."
output: 
  def calculate_rRNA_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_transcripts = 0
      MT_transcripts = 0
      total_reads = 0
      
      with open(htseq_counts_file, 'r') as f:
          for line in f:
              if not line.startswith("__"):
                  gene_id, count = line.strip().split('\t')
                  count = int(count)
                  total_reads += count
                  if gene_id.startswith("rRNA"):
                      rRNA_transcripts += count
                  elif gene_id.startswith("MT"):
                      MT_transcripts += count
      
      rRNA_percentage = (rRNA_transcripts / total_reads) * 100
      MT_percentage = (MT_transcripts / total_reads) * 100
      
      return {
          "rRNA_transcripts": rRNA_transcripts,
          "MT_transcripts": MT_transcripts,
          "rRNA_percentage": round(rRNA_percentage, 3),
          "MT_percentage": round(MT_percentage, 3)
      }
  