[wasmer] unwrap panic when non-digit argument is provided to wasmer

[pventuzelo]

Describe the bug

wasmer panic when giving a string instead of a digit.

./target/release/wasmer run fib.wasm -- abc
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ParseIntError { kind: InvalidDigit }', src/libcore/result.rs:1084:5

Steps to reproduce

Version: 6792db9
Download fib.zip

$ unzip fib.zip
$ ./target/release/wasmer run fib.wasm -- abc
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ParseIntError { kind: InvalidDigit }', src/libcore/result.rs:1084:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

Expected behavior

wasmer should throws an Error.

Actual behavior

RUST_BACKTRACE=1 ../target/release/wasmer run fib.wasm -- abc
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ParseIntError { kind: InvalidDigit }', 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::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
  12: wasmer::execute_wasm
  13: wasmer::main
  14: std::rt::lang_start::{{closure}}
  15: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:49
  16: std::panicking::try::do_call
             at src/libstd/panicking.rs:296
  17: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:80
  18: std::panicking::try
             at src/libstd/panicking.rs:275
  19: std::panic::catch_unwind
             at src/libstd/panic.rs:394
  20: std::rt::lang_start_internal
             at src/libstd/rt.rs:48
  21: main
  22: __libc_start_main
  23: _start
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[pventuzelo]

Merge in master here: c52c44a