Provide meaningful exception codes

[TravisCarden]

Exceptions don't currently follow any particular rule for their exception codes. The default values are used for new exceptions, and wrapping exceptions just use the code of the exception they wrap. As such, they provide no real value, whereas they could be useful in cases such as Handle "Composer not found" error better in ComposerExecutableValidator [#3321966] | Drupal.org, not to mention automated tests.

Settle on an exception code scheme and implement it. Codes should be defined as public constants on exception classes or interfaces, or maybe as enums in PHP 8.