Improved error handling of the calc cg% function

calcgc.py

@@ -21,6 +21,11 @@ def calc_gc_perc(seq):
             atc += 1
         elif c in ('C', 'G'):
             gcc += 1
+        else:
+            raise ValueError(
+                "Unexpected character found: {}. Only "
+                "ATCG are allow.".format(c))
+
     return gcc * 100.0 / (gcc + atc)
 
 inpseq = sys.argv[1]