Enhance error handling with exceptions and add tests

tests/error_data/duplicate_attributes.arff

@@ -0,0 +1,10 @@
+@relation test
+
+@attribute feature1 real
+@attribute feature2 real
+@attribute feature1 real
+@attribute class {A,B}
+
+@data
+1.0,2.0,3.0,A
+4.0,5.0,6.0,B

tests/error_data/empty_attribute_name.arff

@@ -0,0 +1,9 @@
+@relation test
+
+@attribute feature1 real
+@attribute   real
+@attribute class {A,B}
+
+@data
+1.0,2.0,A
+4.0,5.0,B

tests/error_data/empty_attribute_type.arff

@@ -0,0 +1,9 @@
+@relation test
+
+@attribute feature1 real
+@attribute feature2
+@attribute class {A,B}
+
+@data
+1.0,2.0,A
+4.0,5.0,B

tests/error_data/empty_attributes.arff

@@ -0,0 +1,7 @@
+@relation test
+
+% This file has no attributes defined
+
+@data
+1,2,3
+4,5,6

tests/error_data/empty_categorical.arff

@@ -0,0 +1,10 @@
+@relation test
+
+@attribute feature1 {X,Y,Z}
+@attribute feature2 real
+@attribute class {A,B}
+
+@data
+X,2.0,A
+,5.0,B
+Z,8.0,A

tests/error_data/empty_class_label.arff

@@ -0,0 +1,10 @@
+@relation test
+
+@attribute feature1 real
+@attribute feature2 real
+@attribute class {A,B}
+
+@data
+1.0,2.0,A
+4.0,5.0, 
+7.0,8.0,B

tests/error_data/invalid_numeric.arff

@@ -0,0 +1,10 @@
+@relation test
+
+@attribute feature1 real
+@attribute feature2 real
+@attribute class {A,B}
+
+@data
+1.0,2.0,A
+not_a_number,5.0,B
+3.0,4.0,A

tests/error_data/no_data.arff

@@ -0,0 +1,8 @@
+@relation test
+
+@attribute feature1 real
+@attribute feature2 real
+@attribute class {A,B}
+
+@data
+% No actual data samples

tests/error_data/quoted_question_mark.arff

@@ -0,0 +1,10 @@
+@relation test
+
+@attribute feature1 string
+@attribute feature2 real
+@attribute class {A,B}
+
+@data
+"What is this?",2.0,A
+"Another question?",5.0,B
+"No question",8.0,A

tests/error_data/wrong_token_count.arff

@@ -0,0 +1,10 @@
+@relation test
+
+@attribute feature1 real
+@attribute feature2 real
+@attribute class {A,B}
+
+@data
+1.0,2.0,A
+4.0,5.0,6.0,B,extra
+7.0,C