Properly handle exceptions in update worker threads

[jeanlauliac]

Right now it crashes the app, causing other threads to have broken depfile pipes. Ex:

libc++abi.dylib: terminating with uncaught exception of type std::runtime_error
jeanlauliac/minimale git:master* » clang: error: unable to execute command: Broken pipe: 13                                                                                     (50)
clang: error: clang frontend command failed due to signal (use -v to see invocation)
Apple LLVM version 8.1.0 (clang-802.0.42)
Target: x86_64-apple-darwin16.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
clang: note: diagnostic msg: PLEASE submit a bug report to http://developer.apple.com/bugreporter/ and include the crash backtrace, preprocessed source, and associated run script.
clang: note: diagnostic msg:
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PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang: note: diagnostic msg: /var/folders/81/kpc027714hl9xchgsp7sjs400000gn/T/main-d3577a.cpp
clang: note: diagnostic msg: /var/folders/81/kpc027714hl9xchgsp7sjs400000gn/T/main-d3577a.sh
clang: note: diagnostic msg: Crash backtrace is located in
clang: note: diagnostic msg: /Users/jean/Library/Logs/DiagnosticReports/clang_<YYYY-MM-DD-HHMMSS>_<hostname>.crash
clang: note: diagnostic msg: (choose the .crash file that corresponds to your crash)
clang: note: diagnostic msg:

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