CheckWriteHook: Don't allow Noop action

[abhinav]

In #1088, we deprecated the CheckWriteAction enum in favor of the new
fully customizable CheckWriteHook. Unfortunately, this introduced a
minor regression: it's now possible to set log.Fatal to no-op with
WriteThenNoop.

Such a configuration will break code that looks like the following:

f, err := os.Open(..)
if err != nil {
    log.Fatal("Cannot open file", zap.Error(err))
}

// Control flow expects that if we get here,
// f is valid and non-nil.
// That's not the case if Fatal no-ops.

fmt.Println(f.Name())

This change fixes the regression by turning Noops into WriteThenFatal
when logging Fatal log messages. This matches the old behavior.

It further clarifies the documentation for CheckWriteHook so that users
know to call runtime.Goexit or os.Exit in them. It's still possible to
write a custom hook that no-ops the log.Fatal, but it requires a little
extra effort from the user to do so.

[codecov]

Codecov Report

Merging #1089 (ba9689d) into master (71ecc42) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1089   +/-   ##
=======================================
  Coverage   98.36%   98.36%           
=======================================
  Files          49       49           
  Lines        2142     2142           
=======================================
  Hits         2107     2107           
  Misses         27       27           
  Partials        8        8           

Impacted Files	Coverage Δ
options.go	100.00% <ø> (ø)
zapcore/entry.go	93.97% <ø> (ø)
logger.go	96.52% <100.00%> (ø)

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