Index: Improve logging when PDF documents cannot be indexed

[lastzero]

As a user with broken or invalid PDF documents, I want to see an error message in the logs when indexing fails so that I am aware of it and can check the documents manually.

[lastzero]

These changes ensure (1) that the MIME type is always checked in addition to the file extension before creating preview images from PDF documents. Additionally, (2) any errors are logged with the appropriate severity level so that they are visible even when debug or trace log mode is not enabled.

A new preview build will soon be available for testing:

• https://docs.photoprism.app/getting-started/updates/#development-preview

[lastzero]

@graciousgrey To ensure that none of the files that should work are skipped due to the new MIME type check, please retest with PDFs from different sources. Thank you!