Follow-up after #308

Follow-up after #308

**Changes proposed in this PR:**
- Now friendship between layer and network is over
- Move `setProperty(propType)` to public
- Add custom exception in `_error.h`
- Change `setProperty` `std::out_of_range` ->
`exception::invalid_property`
- Add error code boundary to `nntrainer_exception_boundary`
- Change inherited docs to @copydoc

**Self evaluation:**
1. Build test: [X]Passed [ ]Failed [ ]Skipped
2. Run test: [X]Passed [ ]Failed [ ]Skipped

resolves #315

Signed-off-by: Jihoon Lee <jhoon.it.lee@samsung.com>