base64Decode: clearer error message when an invalid character is dete…

…cted

Output the offending string in its entirety to provide context.

Closes #8479

(cherry picked from commit dc3ccf0)

src/libutil/util.cc

@@ -260,8 +260,9 @@ std::string base64Decode(std::string_view s)
         if (c == '\n') continue;
 
         char digit = base64DecodeChars[(unsigned char) c];
-        if (digit == npos)
-            throw Error("invalid character in Base64 string: '%c'", c);
+        if (digit == npos) {
+            throw Error("invalid character in Base64 string: '%c' in '%s'", c, s.data());
+        }
 
         bits += 6;
         d = d << 6 | digit;