[FS#50] No image or non-verbose output created if image is too big

[LEDE Bugs]

The following task has a new comment added:

FS#50 - No image or non-verbose output created if image is too big
User who did this - lemoer (lemoer)

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I think it would be better to introduce a state "image failed", where the build process continues building other images and targets. At the end of the build process, it should print:

"The following images have failed:
- ar71xx-generic
    - TP-Link TL-WR841N/ND v11 (images are too big by 53912 bytes)
    - ...
- ...
"

and exit with unclean exit code.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=50#comment2057