Improve Argument Checking Utilities Coverage

[olivia-banks]

Relevant Codecov:

• https://app.codecov.io/gh/UofUEpiBio/epiworldR/blob/main/R%2Farg-checks.R

To Bind:

• stopifnot_model (failure case)
• stopifnot_agent
• stopifnot_entity (failure case)
• stopifnot_entity_distfun
• stopifnot_tool (failure case)
• stopifnot_tfun
• stopifnot_tool_distfun (failure case)
• stopifnot_virus
• stopifnot_vfun
• stopifnot_virus_distfun

Concordant with #77.