Better UX when TU build fails

[Mrkol]

Currently, if you are viewing a diff of a function and make an edit that results in a compilation error, objdiff will throw you out of the asm view and show the function list on the left and the compilation error on the right. Furthermore, it is now impossible to select a different TU until the compilation error is fixed. All of this is mildly infuriating. Proposed behavior:

1. Compilation error replaces the base asm but leaves the currently selected target asm as-is
2. Pushing escape/back/etc still takes you to the symbol list view AND brings the TU selector back, but shows the error instead of the base symbol list.