add error handling api guidance (#4557)

* add error handling api guidance

docs/readthedocs/source/doc/error-log-api.md

@@ -0,0 +1,109 @@
+BigDL provides error handling api. Please don't use `assert`, `raise`, `throw` to fail the application.
+Use the error handling api instead, it will provide useful message for debugging.
+
+## **Error handling API**
+
+**Scala**
+
+Import package
+```scala
+import com.intel.analytics.bigdl.dllib.utils.Log4Error
+```
+
+If user's input is invalid
+```scala
+Log4Error.invalidInputError(condition: Boolean, errmsg: String, fixmsg: String = null)
+```
+
+* `condition`: Will throw exception and print errmsg if it's `false`. Otherwise will pass the function
+* `errmsg`: Error message to be print.
+* `fixmsg`: Message about how to fix the error.
+
+If user call the api wrong
+```scala
+Log4Error.invalidOperationError(condition: Boolean, errmsg: String, fixmsg: String = null, cause: Throwable = null)
+```
+
+* `condition`: Will throw exception and print errmsg if it's `false`. Otherwise will pass the function
+* `errmsg`: Error message to be print.
+* `fixmsg`: Message about how to fix the error.
+* `cause`: Exception need to throw.
+
+For unkown Exception:
+```scala
+Log4Error.unKnowExceptionError(condition: Boolean, errmsg: String, fixmsg: String = null, cause: Throwable = null)
+```
+
+* `condition`: Will throw exception and print errmsg if it's `false`. Otherwise will pass the function
+* `errmsg`: Error message to be print.
+* `fixmsg`: Message about how to fix the error.
+* `cause`: Exception need to throw.
+
+Notes: This API is for future extension, in case we need distinct invalidOperation exception with unKnownException in python
+
+
+**Python**
+
+Import package
+```python
+from bigdl.dllib.utils.log4Error import *
+```
+
+If user's input is invalid
+```python
+invalidInputError(condition, errMsg, fixMsg=None)
+```
+
+* `condition`: Will throw exception and print errmsg if it's `false`. Otherwise will pass the function
+* `errMsg`: Error message to be print.
+* `fixMsg`: Message about how to fix the error.
+
+If user call the api wrong
+```python
+invalidOperationError(condition, errMsg, fixMsg=None, cause=None)
+```
+
+* `condition`: Will throw exception and print errmsg if it's `false`. Otherwise will pass the function
+* `errMsg`: Error message to be print.
+* `fixMsg`: Message about how to fix the error.
+* `cause`: Exception need to throw.
+
+---
+## **Examples**
+**Scala**
+
+If you want to use:
+```scala
+assert(a > 0)
+```
+or
+```scala
+require(a > 0)
+```
+or
+```scala
+if (a <= 0) {
+  throw new Exception()
+}
+```
+Please use below code instead
+```scala
+Log4Error.invalidInputError(a>0, errmsg="a is negative", fixmsg="expect a is positive")
+```
+
+**Python**
+
+If you want to use:
+```python
+assert(a > 0)
+```
+or
+```python
+if (a <= 0) {
+  raise Exception()
+}
+```
+Please use below code instead
+```python
+invalidInputError(a>0, errMsg="a is negative", fixMsg="expect a is positive")
+```