How dangerous is disclosing a table's structure to users?

What you are referring to is potentially an error-based SQLi vulnerability, which falls under a larger category known as In-band/Classic SQL injection. Essentially, these vulnerabilities allow an attacker to both launch an attack as well as gather information about the database from the same communication channel. I say it is only a potential vulnerability because we are not sure if you've actually tried SQLi, rather you've just discovered an error. This can be confirmed via testing, assuming permission.

The error-based SQLi vulnerability you are potentially seeing provides information about database structure (as you've pointed out) which aids in enumeration of the database (potentially the entire database) for the attacker.

It's saying, "Hello friendly internet user who I have no reason to mistrust, the query you are trying to perform (with obviously no malicious intent) is unfortunately incorrect, so here is the exact structure of my database so that your next query can be more appropriately written to achieve success!"

Such a verbose error message should be disabled on a live site or logged to a file with restricted access. A better practice would be to, depending on the situation, provide the user with either a 400 or 500 error while being cognizant to not provide more information than is absolutely necessary.