Add custom error support for FFI

[jimthematrix]

Add support for specifying custom error definitions in FFI as described in hyperledger/firefly-evmconnect#42

[codecov-commenter]

Codecov Report

Merging #42 (54c51d1) into main (6e072da) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              main       #42     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           39        49     +10     
  Lines         2202      3345   +1143     
===========================================
+ Hits          2202      3345   +1143     

Impacted Files	Coverage Δ
pkg/fftypes/ffi.go	100.00% <ø> (ø)
pkg/i18n/en_base_error_messages.go	100.00% <ø> (ø)
pkg/i18n/en_base_messages.go	100.00% <ø> (ø)
pkg/dbsql/config.go	100.00% <100.00%> (ø)
pkg/dbsql/database.go	100.00% <100.00%> (ø)
pkg/dbsql/filter_sql.go	100.00% <100.00%> (ø)
pkg/dbsql/provider.go	100.00% <100.00%> (ø)
pkg/ffapi/apirequest.go	100.00% <100.00%> (ø)
pkg/ffapi/filter.go	100.00% <100.00%> (ø)
pkg/ffapi/handler.go	100.00% <100.00%> (ø)
... and 5 more

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