Replace "can not" with "cannot"

src/jastadd/ast/AttributeProblems.jrag

@@ -467,7 +467,7 @@ aspect AttributeProblems {
       if (value == null) {
         msg = "missing CollectionGroup argument";
       } else if (value.equals("")) {
-        msg = "CollectionGroup argument can not be empty";
+        msg = "CollectionGroup argument cannot be empty";
       } else if (!isValidIdentifierPart(value)) {
         msg = "CollectionGroup argument must be a valid identifier part";
       }

src/java/org/jastadd/Problem.java

@@ -56,7 +56,7 @@ public abstract class Problem {
 
     public ProblemBuilder sourceFile(String filename) {
       if (filename == null) {
-        throw new NullPointerException("filename can not be null");
+        throw new NullPointerException("filename cannot be null");
       }
       this.filename = filename;
       return this;
@@ -84,7 +84,7 @@ public abstract class Problem {
 
     private void checkNotNull() {
       if (message == null) {
-        throw new NullPointerException("message can not be null");
+        throw new NullPointerException("message cannot be null");
       }
     }
   }