CeylonEnter: new method collectTreeErrors()
CeylonEnter #1055: abort if there were any errors during module resolution

Otherwise we're just begging for trouble and taking time for nothing
model loader #1055: Removed logDuplicateModuleError

We now log it in the tree on the proper import
AbstractModelLoader #1055: added error-checking to package caching wrt modules
LazyModule #1055: visit imports when looking for a module, including transitive exported/shared modules
CeylonEnter #1055: do not run assertions right after module import, or to collect generator errors
LazyModuleManager #1055: log the duplicate module error on the right import node
CMRTest #1055: updated test error messages
RecoveryTest updated error count as a side-effect of #1055: stop after module import phase in case of errors
Test for  #1055: no longer fails