[Buidler] temp files are deleted even when error during build

[RosaWagner]

• The directories master_ufo and instance_ufo should not be deleted if the font doesn't build
• There should not be "Done building fonts!" if the build fails

fontmake: Error: In 'instance_ufo/Borel-Regular.ufo': Compiling UFO failed: instance_ufo/Borel-Regular.ufo/features.fea:126:2: Expected a glyph class with 109 elements after "by", but found a glyph class with 110 elements
ninja: build stopped: subcommand failed.
Removed file: /Users/rosalie/Google/CursiveCP/Borel/sources/build.ninja
Removed file: /Users/rosalie/Google/CursiveCP/Borel/sources/.ninja_log
Removed directory: /Users/rosalie/Google/CursiveCP/Borel/sources/master_ufo
Removed directory: /Users/rosalie/Google/CursiveCP/Borel/sources/instance_ufo
Done building fonts!

[eliheuer]

This was my oversight in PR 721, I'll try to fix this.

[vv-monsalve]

This is still happening in one recent build run.