SPARK-6938: All require statements now have an informative error message.

This pr adds informative error messages to all require statements in the Vectors class that did not previously have them. This references [SPARK-6938](https://issues.apache.org/jira/browse/SPARK-6938).

Author: Juliet Hougland <[email protected]>

Closes apache#5532 from jhlch/SPARK-6938 and squashes the following commits:

ab321bb [Juliet Hougland] Remove braces from string interpolation when not required.
1221f94 [Juliet Hougland] All require statements now have an informative error message.

Author: Juliet Hougland

mllib/src/main/scala/org/apache/spark/mllib/linalg/Vectors.scala

@@ -227,15 +227,16 @@ object Vectors {
    * @param elements vector elements in (index, value) pairs.
    */
   def sparse(size: Int, elements: Seq[(Int, Double)]): Vector = {
-    require(size > 0)
+    require(size > 0, "The size of the requested sparse vector must be greater than 0.")
 
     val (indices, values) = elements.sortBy(_._1).unzip
     var prev = -1
     indices.foreach { i =>
       require(prev < i, s"Found duplicate indices: $i.")
       prev = i
     }
-    require(prev < size)
+    require(prev < size, s"You may not write an element to index $prev because the declared " +
+      s"size of your vector is $size")
 
     new SparseVector(size, indices.toArray, values.toArray)
   }
@@ -309,7 +310,8 @@ object Vectors {
    * @return norm in L^p^ space.
    */
   def norm(vector: Vector, p: Double): Double = {
-    require(p >= 1.0)
+    require(p >= 1.0, "To compute the p-norm of the vector, we require that you specify a p>=1. " +
+      s"You specified p=$p.")
     val values = vector match {
       case DenseVector(vs) => vs
       case SparseVector(n, ids, vs) => vs
@@ -360,7 +362,8 @@ object Vectors {
    * @return squared distance between two Vectors.
    */
   def sqdist(v1: Vector, v2: Vector): Double = {
-    require(v1.size == v2.size, "vector dimension mismatch")
+    require(v1.size == v2.size, s"Vector dimensions do not match: Dim(v1)=${v1.size} and Dim(v2)" +
+      s"=${v2.size}.")
     var squaredDistance = 0.0
     (v1, v2) match {
       case (v1: SparseVector, v2: SparseVector) =>
@@ -518,7 +521,9 @@ class SparseVector(
     val indices: Array[Int],
     val values: Array[Double]) extends Vector {
 
-  require(indices.length == values.length)
+  require(indices.length == values.length, "Sparse vectors require that the dimension of the" +
+    s" indices match the dimension of the values. You provided ${indices.size} indices and " +
+    s" ${values.size} values.")
 
   override def toString: String =
     "(%s,%s,%s)".format(size, indices.mkString("[", ",", "]"), values.mkString("[", ",", "]"))