Output more metadata on expression errors, fix #1440

[sc1f]

This PR changes the validate_expressions API to output an expression error object versus just an error message.

For errors from ExprTk's parser (invalid symbol, mismatched parentheses, etc.), we use ExprTk's update_error method to generate a line and column number for the error. Because we pre-parse the expression strings, on certain expressions the line/column positions of a token that errored might not be totally accurate, as the parsed expression string (COLUMN1 + COLUMN2, vs. "Sales" + "Profit" as typed by the user) will not match 1:1 in terms of token position to the string the user typed. However, without doing a large amount of parsing and other validation, this is good enough for a generalized idea of where an error might be (especially as it does not output a range, just a line and a column number).

Each error object now contains the following keys:

• error_message: a string that describes the error
• line: an integer >= 0 that marks the line the error occurred on
• column: an integer >= that marks the column position the error occurred on

Type errors and "Column does not exist" errors will always return 0 for both line and column.

Additionally, this test cleans up some missing definitions in index.d.ts as reported by #1440.

[texodus]

Looks good! Thanks for the PR!