precompilation: suppress error output when called from loading
As noted in #60148, we currenly dump a bunch of error output to stderr
in our precompile tests (for cases where an error occurs during
precompilation). One easy answer would just be to wrap this in
redirect_stderr. However, it seems bad to me to separate the
actual PrecompileError from the output that explains why the
precompilation failed. So instead, this PR
1. Disables forwarding of output from loading in !fancyprint
mode also.
2. In both fancyprint and non-fancyprint mode, prints any additional
output from *successfully* precompiled packages after precompilation
succeeds, but
3. Puts the output from *failed* precompilations into the error
instead.
I wasn't sure how much I like this separation, but on the other hand
output from successfully-precompiled packages is likely to be
informational, while the output from failed packages is not - it
makes sense to treat these separately.
Fxies #60148
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
