[Spanify] Prevent evaluate_patches from erroring on large debug text

Currently if the errors or diff exceed 50000 characters google returns a
400, stating the maximum is 50000. Since we don't need to worry to much
about the individual error (we can always replicate locally if needed)
we just truncate on the appending so that we get the pass fail result
uploaded even if the diff/error message is truncated.

Change-Id: I9edc76843093acad1bc470f6a441d1dad5bdfe7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6237118
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Auto-Submit: Stephen Nusko <nuskos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1416738}