Error codes

The library reports codes and never messages: it knows nothing about who reads them or in what language. A calling domain maps them onto its own error envelope, and a frontend derives its wording from them.

Code	Meaning
value.required	A required field's cell was empty
value.type-mismatch	The value does not read as the field's type
value.exact-length, value.min-length, value.max-length	A length rule
value.out-of-range	A minimum or maximum
value.not-allowed	Outside the allowed set
value.not-unique	The column repeats a value, and the field identifies a record
value.pattern	Did not match the pattern (as a whole: patterns are anchored at both ends)
group.required	A row carries none of a group's variants
mapping.unknown-field, mapping.duplicate-binding, mapping.required-field-unmapped, mapping.required-group-unmapped	A plan that does not fit its schema
mapping.invalid-column, mapping.invalid-header-row, mapping.invalid-sheet, mapping.unknown-culture	A plan that is malformed
mapping.constraint-type-mismatch	The schema puts a range (MinValue/MaxValue) on a field that is not a number
mapping.stale-profile	The profile was measured against a different header row
mapping.header-changed	The column's header is not the one the mapping recorded
mapping.invalid-plan	MappingPlanException: the plan does not fit its schema; its Faults carry the codes above
structure.sheet-missing, structure.sheet-changed, structure.header-row-missing, structure.header-changed	TabularStructureException: the file is not the one the plan was built for
format.unsupported, format.corrupt, format.truncated	TabularFormatException: not a format this library reads (.xls, .xlsb, .fods, another OpenDocument type, binary, an archive with nothing readable), or damaged, or cut off
limit.exceeded	TabularLimitException: a bound was exceeded; Limit names the option, Maximum its value

This table is checked against the library's sources by ErrorCodeCatalogTests, in both directions. It went out of step twice in the branch that added it — a code emitted, asserted, given a requirement and described in this file's own prose, and left out of the table a frontend reads. Now it cannot.

Faults that invalidate a whole run are exceptions, not row errors — the two demand opposite responses. All of them derive from TabularException, which carries a Code from the table, and split by what a host does about them:

Exception	Means	Typical HTTP answer
TabularFormatException	Not a file this library reads, or not a readable one	400 / 415
TabularLimitException	Readable, but beyond a configured bound — how most hostile files end	413
TabularStructureException	Not the file the plan was built for (sheet, header row or header changed)	409 / 422
MappingPlanException	The plan does not fit its schema, before any file is read	400

Mistakes in the calling code — a null argument, an option out of range, a field the schema does not declare — are ArgumentException and InvalidOperationException. Nothing else escapes: malformed XML and a damaged zip are reported as TabularFormatException with the parser's error as the inner exception.