chore(snowplow): Add support for parsing referrer URIs, improved error strings

This MR brings a set of changes, each broken down into its own (single) commit to make reviewing easier, namely:

• Add support for populating Snowplow fields that depend on passed referrer URI.
• Improve error strings generated during enrichment errors, aligning it with upstream wherever possible.
• Initialise compiled regex into a package variable.
• Use uuid.Validate() to validate incoming UUIDs instead of uuid.Parse().

These changes were also developed and tested with the ongoing effort in 8ed18813.

Related: #100.