input: Write a function to calculate the percent of transcripts mapping to rRNA from an HTseq_counts file and an original reference transcriptome aligned to. The function should take two arguments: the path to the HTseq_counts file and the path to the GTF file. The function should return the percent of rRNA mapped reads and the percent of MT mapped reads as a tuple. If any of the files do not exist, the function should return an error message.
  assert calculate_rRNA_percent("C17_3_S20_Aligned.out.srt.bam_htseq_combined_counts.txt", "/Users/malwina/Documents/CTR-Data/genomes/Mus_musculus/mm10/Mus_musculus.GRCm38.84.gtf")==("rRNA: 5.574%", "MT: 2.345%")
  assert calculate_rRNA_percent("non_existent_file.txt", "/Users/malwina/Documents/CTR-Data/genomes/Mus_musculus/mm10/Mus_musculus.GRCm38.84.gtf")=="Error: File non_existent_file.txt does not exist"
  assert calculate_rRNA_percent("C17_3_S20_Aligned.out.srt.bam_htseq_combined_counts.txt", "non_existent_file.gtf")=="Error: GTF File non_existent_file.gtf does not exist"
output: 
  def calculate_rRNA_percent(htseq_counts_path, gtf_path):
      import os
      if not os.path.exists(htseq_counts_path):
          return f"Error: File {htseq_counts_path} does not exist"
      if not os.path.exists(gtf_path):
          return f"Error: GTF File {gtf_path} does not exist"
      
      rRNA_count = 0
      MT_count = 0
      total_count = 0
      
      with open(htseq_counts_path, 'r') as htseq_file:
          for line in htseq_file:
              if line.startswith("__"):
                  continue
              count = int(line.split()[1])
              total_count += count
              if "rRNA" in line:
                  rRNA_count += count
              if "MT" in line:
                  MT_count += count
      
      rRNA_percent = (rRNA_count / total_count) * 100
      MT_percent = (MT_count / total_count) * 100
      
      return f"rRNA: {rRNA_percent:.3f}%", f"MT: {MT_percent:.3f}%"
  