Improve error handling and reporting

[welpo]

Summary

The proposed changes introduce enhanced error handling mechanisms to the theme processing code. The revised script gathers errors during execution and provides a summarised report at the end.

Related issue: #119

Main Changes

1. Centralized error collection: Introduces an errors list to gather all the issues detected during script execution.
2. Graceful failure on TOML parsing: Previously, failing to parse a theme's TOML metadata would result in a crash. Now, it captures the error, logs it, and allows the script to continue processing other themes.
3. Enhanced reporting: At the end of the script's run, all collected errors are displayed in a formatted manner, offering a clear view of issues to address.
4. Script summary: After processing, the script provides a count of the total themes successfully processed and the number of themes with errors.

Example

Here's a sample of the new error summary:

============================================================
ERROR SUMMARY:
------------------------------------------------------------
Theme 'abc' is missing required metadata: name.
------------------------------------------------------------
Theme 'def' is missing screenshot.png.
------------------------------------------------------------
Theme 'ghi' is missing README.md.
------------------------------------------------------------
Theme 'jkl' encountered a TOML parsing issue: Duplicate keys! (line 5 column 1 char 121)
------------------------------------------------------------
============================================================


Themes successfully processed: 68
Themes with errors: 4

Benefits

• Enhanced clarity: Problematic themes become instantly identifiable, minimising the chance of oversight.
• Resilience: The script's enhanced error handling ensures that a single problematic theme.toml doesn't halt the entire process.

[Keats]

Very nice, thanks!