[lld/WebAssembly] Slightly nicer error message for malformed input files

Summary:
Before:
```
wasm-ld: error: Relocations not in offset order
```
After
```
wasm-ld: error: While processing `libjulia.so`: Relocations not in offset order
```

At least this way you get to find out which input file is malformed.

Reviewers: sbc100
Differential Revision: https://reviews.llvm.org/D63694

llvm-svn: 364368