NPE when referencing undefined fragment

[bannmann]

Referencing a fragment that is not defined (e.g. due to a typo) causes the following NPE:

java.lang.NullPointerException
    at silverchain.parser.adapter.ASTBuilder.visitFragmentReference (ASTBuilder.java:454)
    at silverchain.parser.adapter.ASTBuilder.visitRuleAtom (ASTBuilder.java:214)
    at silverchain.parser.adapter.ASTBuilder.visitRuleElement (ASTBuilder.java:199)
    at silverchain.parser.adapter.ASTBuilder.visitRuleFactor (ASTBuilder.java:188)
    at silverchain.parser.adapter.ASTBuilder.visitRuleTerm (ASTBuilder.java:178)

Silverchain should produce a readable error message in this case, e.g. "$FOO is not defined at #L1C2".

[bannmann]

Note: the stack trace was produced by a 0fab283 build of Silverchain.