Clearer errors · onelite docs

OneLite's errors say the same thing SQLite-compatible libraries say, and then say more. Nothing about the default behaviour changes.

What stays identical

OneLiteException.Message, Code (the primary result code) and ExtendedCode equal what the pinned SQLite release reports for the same statement on the same data: the same text, the same codes. OneLiteDataException.Message is SQLite Error <code>: '<message>'. as before. A test (tests/OneLite.Tests/Diagnostics/ErrorDiagnosticsTests.cs) runs more than 150 failing statements (syntax, names, arity, constraints, STRICT types, misuse) on OneLite and on the pinned library and compares code, extended code and message.

What is added

All members are read-only properties of OneLiteException (and are repeated on OneLiteDataException). They are empty or null when the error has nothing to add. They are filled in only when an error happens, so a statement that succeeds pays nothing.

Member	Meaning
`Sql`	The statement text the error is about.
`Position`	`SqlPosition`: UTF-16 `Offset` and `Length` of the offending text, 1-based `Line` and `Column` (UTF-16 characters), and `ByteOffset` (UTF-8, the unit `sqlite3_error_offset` uses).
`Caret`	The line holding the position, then a line with `^~~` under it. Long lines are cut around the position.
`Identifier`	The name the error is about: the unknown table, column or function, the ambiguous column, the token a syntax error stopped at, the constrained column.
`Suggestions`	Up to three names close to what was written, closest first (see below).
`Constraint`	For a constraint failure: `Kind` (unique, primary key, not null, check, foreign key, STRICT data type), `Table`, `Columns`, `Name` (a CHECK's text) and, for unique and primary-key failures, `KeyValues`, the clashing key as SQL literals.
`Explain()` / `ToString()`	A readable account: the message with its line and column, the SQL line with the caret, the constraint details, "did you mean". `ToString()` is the usual exception text followed by this.

Example:

no such column: nme
at line 1, column 8: no such column: nme
  select nme from users
         ^~~
did you mean "name"?

How the pieces are found

Position. A syntax error uses the position the parser reports. For a name error, the statement text is tokenised again and the first token that spells the name in the role the message implies is taken (after table. when the message names a table; followed by ( for a function; after COLLATE for a collation). A name that appears several times in the statement can therefore be reported at its first use.
Suggestions. Candidates come from the schema in scope: table and view names plus the names a WITH clause defines (unknown table), the columns of the tables the statement mentions (unknown column), the registered scalar function names and the aggregate and window function names (unknown function), the three built-in collations, and the common SQL keywords (a syntax error that stopped at a misspelt keyword). A candidate counts when its edit distance (insertions, deletions, substitutions, swaps of neighbours; case ignored) is at most a third of the written name's length (at least 1, at most 3), when the written name is a prefix of it, or, for names of two or more letters, when its letters appear in order inside the candidate (nm suggests name). For an unknown column that another table has, the suggestion is table.column; for an ambiguous column, the list of table.column that have it. For a wrong number of arguments, the single suggestion says how many the function takes.
Constraints. Kind, table and columns come from the message. The key values are copied from the registers that held the new row's key when the uniqueness probe failed, and are formatted only on the error path.

Limits

Runtime errors that are not constraint failures (an integer overflow, a bad LIMIT) carry the statement text but no position in it: the virtual machine does not track which expression a failing instruction came from. A name that does not appear in the statement text itself (for example one inside a view body) has no position. Suggestions never change which error is reported.