Details in error messages

[kparichay]

Error messages should contain enough details to point out the location of error:

• Layer/LayerNode/LayerContext -> must include layer name in the error message
• Optimizer/OptimizerContext -> must include optimizer and weight name in the error message
• Tensor/VarGrad/Weight -> must include tensor name in the error message
• Classes -> must include class name and the high level task at which the error message cam
• *Later we can consider providing a call stack in the error message with debug mode

Apart from the correct identifier, possible hints/solution must be also added to the error message:

• give possible reasons why this error has occured (from the user side or from the internal behavior)
• give solutions for solving the error

[taos-ci]

:octocat: cibot: Thank you for posting issue #1803. The person in charge will reply soon.

[zhoonit]

additional list

• fc check if unit is empty at finalize, and issue proper warning