[wasmer binary] panic when target module don't have exported _start function

[pventuzelo]

Describe the bug

wasmer panic when using the loader parameter associated with a module without _start exported function
version: fed80fc

echo "`wasmer -V` | `rustc -V` | `uname -m`"
wasmer 0.7.0 | rustc 1.38.0-nightly (ad7c55e1f 2019-08-07) | x86_64

Steps to reproduce

Download fib.zip
Run: ./target/release/wasmer run --loader local fib.wasm

Expected behavior

wasmer should return the same error message but without panicking.

Actual behavior

wasmer crash with the following message:

thread 'main' panicked at 'The loader requires a _start function to be present in the module: ExportNotFound { name: "_start" }', src/libcore/result.rs:1084:5

Additional context

RUST_BACKTRACE=1 ./target/release/wasmer run --loader local fib.wasm 
thread 'main' panicked at 'The loader requires a _start function to be present in the module: ExportNotFound { name: "_start" }', src/libcore/result.rs:1084:5
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:200
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:214
   6: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:477
   7: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:384
   8: rust_begin_unwind
             at src/libstd/panicking.rs:311
   9: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
  10: core::result::unwrap_failed
             at src/libcore/result.rs:1084
  11: core::result::Result<T,E>::expect
             at /rustc/ad7c55e1fc55d9af4787b285cec1c64e3480ae84/src/libcore/result.rs:879
  12: wasmer::execute_wasm
             at src/bin/wasmer.rs:528
  13: wasmer::run
             at src/bin/wasmer.rs:757
  14: wasmer::main
             at src/bin/wasmer.rs:825
  15: std::rt::lang_start::{{closure}}
             at /rustc/ad7c55e1fc55d9af4787b285cec1c64e3480ae84/src/libstd/rt.rs:64
  16: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:49
  17: std::panicking::try::do_call
             at src/libstd/panicking.rs:296
  18: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:80
  19: std::panicking::try
             at src/libstd/panicking.rs:275
  20: std::panic::catch_unwind
             at src/libstd/panic.rs:394
  21: std::rt::lang_start_internal
             at src/libstd/rt.rs:48
  22: main
  23: __libc_start_main
  24: _start
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.