feat(hogql): better error if placeholder in HogQL expression

[mariusandra]

Problem

• Stacked on top of feat(hogql): select statements #14131
• Part of HogQL & Data Exploration next steps meta#81
• Followup in feat(hogql): symbol resolution #14185

Changes

Throws a slightly nicer looking error when accidentally using {placeholder} syntax within HogQL expressions, for example in insights or in the events table.

The previous error looked like we had explicitly forgotten to add a placeholder, while it was actually bad data. This makes it clear that no placeholders are allowed in user entered strings.

It's all part of a work in progress though...

How did you test this code?

Added a test.

[mariusandra]

Now that #14131 is in (thanks @Twixes), there's this PR that's stacked on top. Might as well get it in, as all followup work is now taking place in #14185

[thmsobrmlr]

Looks great!