Add a custom error message for a missing alias after 'aggregate:'

[mbooth4]

Fixes #34

I added a helpful error message: "'aggregate:' entries must include a name (ex: some_name is count())"

Add an error message rewriter for the case:

run: x -> {
  aggregate: count()
}

Was:

Now:

I also wanted to make sure we could support the aggregate error even when it is nested inside of a view. To do so, we couldn't rely on the same logic because the parser fails to parse the entire view: clause. Instead, I added a new error rewriter option lookbackFromOffendingSymbol that lets us do relative sequence comparisons from the token associated with the error, rather than the token associated with the parser's current position. The Malloy grammar has a lot of ambiguity that is resolved by lookahead logic in the ATN and so we cannot rely on the currentToken to be up-to-date when an error is hit.

source: airports is duckdb.table('../data/airports.parquet') extend {
  measure: airport_count is count()

  view: by_state is {
    where: state is not null
    aggregate: count()
  }
}

Was:

Now: