Refactor Pegasus Plugin to re-integrate count checks back into basic checks.
The primary reason for this is so error messages can emmit all bad/incorrect file pairs,
whether they are two different types (idls or snapshots) or for two different reasons
(missing or changed).
Also changed the emitted error message so that it contains more information and will
emit a special error if it suspects it is being run in a continuous integration environment,
so as not to confuse developers.

RB=214848
R=gogenc,jbetz,si-dev,kjin
A=gogenc,kjin