[REST Server] Show clear error message

[666lcz]

The user will get a vague message if they forget to run sui genesis before ./rest_sever:

➜ ./rest_server
Error: No such file or directory (os error 2)

After this PR:

➜ ./rest_server
Error: Failed to read config file at "/Users/cl/.sui/sui_config/gateway.conf": No such file or directory (os error 2). 
Have you run `sui genesis` first?

[codecov]

Codecov Report

Merging #1336 (1a965d2) into main (4d09cb9) will increase coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1336      +/-   ##
==========================================
+ Coverage   81.72%   81.77%   +0.04%     
==========================================
  Files         100      100              
  Lines       20966    20972       +6     
==========================================
+ Hits        17134    17149      +15     
+ Misses       3832     3823       -9     

Impacted Files	Coverage Δ
sui/src/rest_server.rs	23.43% <0.00%> (-0.34%)	⬇️
sui_core/src/authority/temporary_store.rs	83.52% <0.00%> (-0.40%)	⬇️
...i_programmability/verifier/src/id_leak_verifier.rs	91.03% <0.00%> (ø)
sui_core/src/authority_aggregator.rs	87.43% <0.00%> (+0.72%)	⬆️
sui_core/src/safe_client.rs	60.00% <0.00%> (+4.76%)	⬆️

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