[ASTImporter] Propagate error from ImportDeclContext

Summary:
During analysis of one project we failed to import one
CXXDestructorDecl. But since we did not propagate the error in
importDeclContext we had a CXXRecordDecl without a destructor. Then the
analyzer engine had a CallEvent where the nonexistent dtor was requested
(crash).

Solution is to propagate the errors we have during importing a
DeclContext.

Reviewers: a_sidorin, a.sidorin, shafik

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63603

llvm-svn: 364752