fix(rpc): fix error message for unstaked entity mempool count

[dancoombs]

Fixes #740

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 57.27%. Comparing base (ac74140) to head (2752810).

Additional details and impacted files

Files	Coverage Δ
crates/rpc/src/eth/error.rs	0.00% <ø> (ø)
crates/types/src/pool/error.rs	0.00% <ø> (ø)
crates/pool/src/mempool/uo_pool.rs	94.94% <50.00%> (ø)
crates/pool/src/server/remote/error.rs	14.49% <0.00%> (ø)

Flag	Coverage Δ
unit-tests	57.27% <20.00%> (ø)

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Components	Coverage Δ
rundler binary	0.00% <ø> (ø)
builder	59.16% <ø> (ø)
dev	0.00% <ø> (ø)
pool	64.95% <20.00%> (ø)
provider	16.10% <ø> (ø)
rpc	25.64% <ø> (ø)
sim	78.25% <ø> (ø)
tasks	∅ <ø> (∅)
types	66.68% <ø> (ø)
utils	18.28% <ø> (ø)