Report an error when an `UNKNOWN_TAG` is found

[yash-nisar]

Steps to reproduce:

• When I modify any tag, e.g SPDXVersion: SPDX-1.2 to SPDXVersio: SPDX-1.2, the tool should report an error like Found unknown tag : SPDXVersio at line: 2 but instead it continues to parse.

The tokenizing works fine, the problem is with the parsing method.

[yash-nisar]

A better way to reproduce the bug would be:

• On adding an unwanted line like:

SPDXVersion: SPDX-1.2
SPDXVersio: SPDX-1.2

the parsing should emit an error, but it continues to parse without raising any error.

[meretp]

This issue is solved and can be closed. @nicoweidner