Improve template issue forensics.

[theengineear]

The major changes can be boiled down into the following additions:

• Print the registered custom element tag name in the error.
• Print the approximate template line number in the error.†

† Note that this isn’t the file line number, just a naive count of the number of newlines encountered so far in the given template.

Closes #201

[theengineear]

@klebba — I was kicking the tires again on migrating to the default template engine. These changes make the process much more efficient.

[theengineear]

@klebba — Gonna get this one merged in based on our conversation earlier 🤙