dartdoc should have a better error message for invalid reexports

[jcollins-g]

Problems in the analyzer in resolving export directives due to missing files currently result in ugly exceptions. Convert this to a dartdoc error (or possibly, warning) instead that gives an indication of where the problem is, and keep going on doc generation.

Generation failed: NoSuchMethodError: The getter 'exports' was called on null.
Receiver: null
Tried calling: exports
dart:core                               Object.noSuchMethod
package:dartdoc/src/model.dart 4405:58  PackageGraph._tagReexportsFor
package:dartdoc/src/model.dart 4406:7   PackageGraph._tagReexportsFor
package:dartdoc/src/model.dart 4406:7   PackageGraph._tagReexportsFor
package:dartdoc/src/model.dart 4417:9   PackageGraph.libraryElementReexportedBy
package:dartdoc/src/model.dart 4536:9   PackageGraph.findLibraryFor
package:dartdoc/src/model.dart 4724:28  PackageGraph.findOrCreateLibraryFor
package:dartdoc/src/model.dart 4050:7   new PackageGraph.<fn>
dart:collection                         __CompactLinkedHashSet&_HashFieldBase&_HashBase&_OperatorEqualsAndHashCode&SetMixin.forEach
package:dartdoc/src/model.dart 4049:28  new PackageGraph
package:dartdoc/src/model.dart 5511:16  PackageBuilder.buildPackageGraph
===== asynchronous gap ===========================
dart:async                              _AsyncAwaitCompleter.completeError
package:dartdoc/src/model.dart          PackageBuilder.buildPackageGraph
===== asynchronous gap ===========================
dart:async                              _asyncThenWrapperHelper
package:dartdoc/src/model.dart          PackageBuilder.buildPackageGraph
package:dartdoc/dartdoc.dart 139:26     Dartdoc.generateDocs

[jcollins-g]

I think a solution to this should keep track of the recursive _tagReexportsFor calls and print a trace of the exports traveled from the original LibraryElement to the one that is null.