input: Write a function to calculate the percent of transcripts mapping to rRNA given an HTseq_counts file and a GTF file. If the files do not exist, raise an error and exit the program. The function should return the summary file name and the percentage of rRNA and MT mapped reads.
  assert calculate_rrna_percent("/path/to/HTseq_counts/file", "/path/to/GTF/file")==("HTseq_counts_file_rRNAmtRNACounts.txt", "xx.xx", "yy.yy")
  assert calculate_rrna_percent("/invalid/path", "/path/to/GTF/file")=="Error->\tFile: /invalid/path does not exist"
  assert calculate_rrna_percent("/path/to/HTseq_counts/file", "/invalid/path")=="Error->\tGTF File: /invalid/path does not exist"
output: def calculate_rrna_percent(htseq_counts_file, gtf_file):
      import os
      if not os.path.exists(htseq_counts_file):
          return f"Error->\tFile: {htseq_counts_file} does not exist"
      if not os.path.exists(gtf_file):
          return f"Error->\tGTF File: {gtf_file} does not exist"
      
      rrna_count = 0
      mt_count = 0
      total_count = 0
      
      with open(htseq_counts_file, 'r') as f:
          for line in f:
              if line.startswith("__"):
                  continue
              parts = line.strip().split()
              if len(parts) == 2:
                  gene_id, count = parts
                  count = int(count)
                  total_count += count
                  if "rRNA" in gene_id:
                      rrna_count += count
                  if "MT" in gene_id:
                      mt_count += count
      
      rrna_percent = (rrna_count / total_count) * 100 if total_count > 0 else 0
      mt_percent = (mt_count / total_count) * 100 if total_count > 0 else 0
      
      summary_file_name = f"{os.path.basename(htseq_counts_file)}_rRNAmtRNACounts.txt"
      
      return summary_file_name, f"{rrna_percent:.2f}", f"{mt_percent:.2f}"