Make the instructions for binary more ambiguous

Some languages treat invalid values as 0, others return errors, yet others raise exceptions.
We should allow for whatever error handling is idiomatic in the language.

binary.md

@@ -1,10 +1,10 @@
-Implement binary to decimal conversion.  Given a binary input
-string, your program should produce a decimal output.
+Implement binary to decimal conversion. Given a binary input
+string, your program should produce a decimal output. The
+program should handle invalid inputs.
 
 ## Note
 - Implement the conversion yourself.
   Do not use something else to perform the conversion for you.
-- Treat invalid input as binary 0.
 
 ## About Binary (Base-2)
 Decimal is a base-10 system.