AssetServer LoadState API consistency (#15237)
# Objective
- implements consistently named AssertServer methods for asset,
dependency, and recursive dependency load states
- returns relevant LoadState when required, including error information
for failed loads
- resolves #15098
## Solution
- implement consistently named LoadState accessor methods:
- load_state, dependency_load_state, recursive_dependency_load_state
(return unwrapped load states)
- get_load_state, get_dependency_load_state,
get_recursive_dependency_load_state (return Option)
- is_loaded, is_loaded_with_dependencies,
is_loaded_with_recursive_dependencies (return bool)
- adds AssetLoadError to DependencyLoadState::Failed and
RecursiveDependencyLoadState::Failed
## Testing
- Added coverage to existing unit tests