add more info when c2t files modified more recently than .fa

see #64. thanks @rmoran7

bwameth.py

@@ -346,7 +346,7 @@ def bwa_mem(fa, fq_convert_cmd, extra_args, threads=1, rg=None,
         sys.stderr.write("Found BWA MEM2 index\n")
 
     else:
-        raise BWAMethException("first run bwameth.py index %s OR bwameth.py index-mem2 %s" % (fa, fa))
+        raise BWAMethException("first run bwameth.py index %s OR bwameth.py index-mem2 %s OR make sure the modification time on the generated c2t files is newer than on the .fa file" % (fa, fa))
 
 
     if not rg is None and not rg.startswith('@RG'):