Make ErrHashStrSize slightly more descriptive.

Add the word 'string' to the error string so if this is printed, it
will be clearer that it was a hash string decode that failed (as
opposed to other errors creating a ShaHash from a byte slice) and
change 'chars' to 'bytes', since the string length is measured in
bytes, not UTF-8 code points.

ok @davecgh

shahash.go

@@ -18,7 +18,7 @@ const MaxHashStringSize = HashSize * 2
 
 // ErrHashStrSize describes an error that indicates the caller specified a hash
 // string that has too many characters.
-var ErrHashStrSize = fmt.Errorf("max hash length is %v chars", MaxHashStringSize)
+var ErrHashStrSize = fmt.Errorf("max hash string length is %v bytes", MaxHashStringSize)
 
 // ShaHash is used in several of the bitcoin messages and common structures.  It
 // typically represents the double sha256 of data.