Replace assertion with exception for unit test network

[hewison-chris]

When makeClient or makeRegistryClient fails then throw an exception as the calling code will handle it. This is an attempt to prevent the Trying to access ... without first creating it test failures.

[codecov[bot]]

Codecov Report

Merging #3279 (311ab9c) into v0.x.x (9d28f5c) will increase coverage by 0.11%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           v0.x.x    #3279      +/-   ##
==========================================
+ Coverage   87.83%   87.95%   +0.11%     
==========================================
  Files         164      165       +1     
  Lines       16932    17044     +112     
==========================================
+ Hits        14873    14991     +118     
+ Misses       2059     2053       -6     

Flag	Coverage Δ
integration	34.25% <ø> (?)
unittests	87.94% <0.00%> (+0.10%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
source/agora/test/Base.d	80.67% <0.00%> (-0.32%)	:arrow_down:
source/scpd/scp/SCPDriver.d	60.00% <0.00%> (-15.00%)	:arrow_down:
source/agora/test/ValidatorRecurringEnrollment.d	94.23% <0.00%> (-0.97%)	:arrow_down:
source/agora/node/main.d	44.85% <0.00%> (ø)
source/agora/script/Engine.d	97.74% <0.00%> (+0.15%)	:arrow_up:
source/agora/network/Manager.d	78.48% <0.00%> (+0.31%)	:arrow_up:
source/agora/consensus/state/Ledger.d	90.72% <0.00%> (+0.33%)	:arrow_up:
source/agora/consensus/protocol/Nominator.d	91.63% <0.00%> (+0.53%)	:arrow_up:
source/agora/node/FullNode.d	74.30% <0.00%> (+0.61%)	:arrow_up:
source/agora/node/Validator.d	92.96% <0.00%> (+1.55%)	:arrow_up:
... and 5 more

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