Error classes associated with individual stages of
dashboard processing tend to have very long names.
As dashboard post-processing includes more steps,
we will likely need to handle more error cases.
Refactoring to have all errors inherit from a specific
base class will help accommodate this and keep the code
more readable.

Adds a new stage to dashboard processesing step for the
EnvironmentsController::metrics_dashboard endpoint.

Allows the front end to avoid generating the endpoint
unitutive string mutations.

Updates the EnvironmentController#metrics_dashboard endpoint
to support a "dashboard" param, which can be used to specify
the filepath of a dashboard configuration from a project
repository. Dashboard configurations are expected to be
stored in .gitlab/dashboards/.

Updates dashboard post-processing steps to exclude custom
metrics, which should only display on the system dashboard.