Fix null pointer crash and incorrect error statuses

[haydentherapper]

The gRPC and legacy servers did not check if the public
key was provided before accesses it, causing a null pointer exception if
no public key was provided in the request.

The errors from the gRPC server to the legacy server were being wrapped,
and the original error status was being lost.

Signed-off-by: Hayden Blauzvern hblauzvern@google.com

Summary

Ticket Link

Fixes #525

Release Note

Fixed null pointer crash and incorrect error statuses from gRPC server

[codecov-commenter]

Codecov Report

Merging #526 (43878c1) into main (d834c55) will increase coverage by 0.31%.
The diff coverage is 30.00%.

❗ Current head 43878c1 differs from pull request most recent head 4c2f7a7. Consider uploading reports for the commit 4c2f7a7 to get more accurate results

@@            Coverage Diff             @@
##             main     #526      +/-   ##
==========================================
+ Coverage   33.43%   33.75%   +0.31%     
==========================================
  Files          18       18              
  Lines        1352     1357       +5     
==========================================
+ Hits          452      458       +6     
  Misses        836      836              
+ Partials       64       63       -1     

Impacted Files	Coverage Δ
pkg/api/legacy_server.go	0.00% <0.00%> (ø)
pkg/api/grpc_server.go	45.73% <100.00%> (+1.01%)	⬆️
pkg/ca/fileca/load.go	68.96% <0.00%> (+10.34%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d834c55...4c2f7a7. Read the comment docs.

[bobcallaway]

Good catch; should we also add required field options in fulcio_legacy.proto?

[haydentherapper]

Good idea, added!