Improve illegal bucket name error message (#2263)

This may help users debug situations like:

https://stackoverflow.com/questions/76359564/why-does-mounting-s3fs-bucket-on-centos-7-using-fstab-fail-but-mount-a-works

src/s3fs_cred.cpp

@@ -1536,7 +1536,7 @@ bool S3fsCred::CheckForbiddenBucketParams()
     // check bucket name for illegal characters
     size_t found = bucket_name.find_first_of("/:\\;!@#$%^&*?|+=");
     if(found != std::string::npos){
-        S3FS_PRN_EXIT("BUCKET %s -- bucket name contains an illegal character.", bucket_name.c_str());
+        S3FS_PRN_EXIT("BUCKET %s -- bucket name contains an illegal character: '%c' at position %zu", bucket_name.c_str(), bucket_name[found], found);
         return false;
     }