Improve the error message on memory region conflict.

Improve the error message on memory region conflict.

Before;
```
ValueError: Memory region conflict between rom and main_ram
```

After;
```
ValueError: Memory region conflict between rom (<SoCMemRegion 0x10000000 0x10000 cached>) and main_ram (<SoCMemRegion 0x0 0x20000000 cached>)
```

Fixes #296.