feat: improve error message for CloudEvent

[coryan]

When parsing CloudEvents in JSON format the error message was a bit
obscure, it indicated that a field was missing, but not which one. This
is more readable in the common case.

[codecov]

Codecov Report

Merging #310 (b146449) into main (bf2ba9a) will decrease coverage by 33.31%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main     #310       +/-   ##
===========================================
- Coverage   88.79%   55.47%   -33.32%     
===========================================
  Files          88      562      +474     
  Lines        2303    15105    +12802     
===========================================
+ Hits         2045     8380     +6335     
- Misses        258     6725     +6467     

Impacted Files	Coverage Δ
...cloud/functions/internal/parse_cloud_event_json.cc	100.00% <100.00%> (ø)
...ux/include/boost/asio/impl/multiple_exceptions.ipp	0.00% <0.00%> (ø)
...ude/boost/archive/iterators/dataflow_exception.hpp	0.00% <0.00%> (ø)
.../include/boost/algorithm/string/classification.hpp	0.00% <0.00%> (ø)
...nclude/google/cloud/spanner/internal/tuple_utils.h	0.00% <0.00%> (ø)
...nux/include/boost/algorithm/string/detail/util.hpp	0.00% <0.00%> (ø)
...64-linux/include/boost/beast/core/detail/tuple.hpp	100.00% <0.00%> (ø)
...ux/include/google/cloud/storage/download_options.h	0.00% <0.00%> (ø)
...x/include/boost/program_options/value_semantic.hpp	77.77% <0.00%> (ø)
...e/boost/intrusive/detail/default_header_holder.hpp	100.00% <0.00%> (ø)
... and 465 more

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