Checking UsageFile: don't fail if the file doesn't exist, just recompile
Checking UsageFile: don't fail if the file doesn't exist, just recompile

If a file we depended on last time is missing, we should recompile.

This also makes us insensitive to mistakes when recording dependent
source files (such as storing a temporary file), but will make more
recompilation happen instead.  With DEBUG on, you get a warning.

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