[BOLT] Check MCContext errors

Abort on emission errors to prevent a malformed binary being written.
Example:
```
<unknown>:0: error: Undefined temporary symbol .Ltmp26310
<unknown>:0: error: Undefined temporary symbol .Ltmp26311
<unknown>:0: error: Undefined temporary symbol .Ltmp26312
<unknown>:0: error: Undefined temporary symbol .Ltmp26313
<unknown>:0: error: Undefined temporary symbol .Ltmp26314
<unknown>:0: error: Undefined temporary symbol .Ltmp26315
BOLT-ERROR: Emission failed.
```

Reviewed By: yota9

Differential Revision: https://reviews.llvm.org/D123263