base64Decode: clearer error message when an invalid character is detected

Output the offending string in its entirety to provide context.

(cherry picked from commit dc3ccf02bfd4d359228b54f5c24ae2b6caf6428e)

src/libutil/strings.cc

@@ -182,7 +182,7 @@ std::string base64Decode(std::string_view s)
 
         char digit = base64DecodeChars[(unsigned char) c];
         if (digit == npos)
-            throw Error("invalid character in Base64 string: '%c'", c);
+            throw Error("invalid character in Base64 string: '%c' in '%s'", c, s.data());
 
         bits += 6;
         d = d << 6 | digit;