[error] Fix wrong error message when using vector as if condition

[archibate]

Related issue = #

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It seems that when pybind11 got an type mismatch, it will try to invoke repr to show the object...
So we have to make it happy with a dummy string in Matrix.__repr__ instead of cannot be called from Python-scope.

import taichi as ti
ti.init()

@ti.kernel
def func():
    x = ti.Vector([2, 3])
    if x:
        pass

func()

Before:

Traceback (most recent call last):
  File "w.py", line 12, in <module>
    func()
  File "/root/taichi/python/taichi/lang/kernel.py", line 553, in wrapped
    return primal(*args, **kwargs)
  File "/root/taichi/python/taichi/lang/kernel.py", line 483, in __call__
    self.materialize(key=key, args=args, arg_features=arg_features)
  File "/root/taichi/python/taichi/lang/kernel.py", line 363, in materialize
    taichi_kernel = taichi_kernel.define(taichi_ast_generator)
  File "/root/taichi/python/taichi/lang/util.py", line 210, in wrapped
    assert in_python_scope(), \
AssertionError: __repr__ cannot be called in Taichi-scope

After:

Traceback (most recent call last):
  File "w.py", line 12, in <module>
    func()
  File "/root/taichi/python/taichi/lang/kernel.py", line 553, in wrapped
    return primal(*args, **kwargs)
  File "/root/taichi/python/taichi/lang/kernel.py", line 483, in __call__
    self.materialize(key=key, args=args, arg_features=arg_features)
  File "/root/taichi/python/taichi/lang/kernel.py", line 363, in materialize
    taichi_kernel = taichi_kernel.define(taichi_ast_generator)
  File "/root/taichi/python/taichi/lang/kernel.py", line 360, in taichi_ast_generator
    compiled()
  File "w.py", line 8, in func
    if x:
  File "/root/taichi/python/taichi/lang/expr.py", line 29, in __init__
    self.ptr = make_constant_expr(arg).ptr
  File "/root/taichi/python/taichi/lang/util.py", line 200, in wrapped
    return func(*args, **kwargs)
  File "/root/taichi/python/taichi/lang/impl.py", line 224, in make_constant_expr
    return Expr(taichi_lang_core.make_const_expr_f32(val))
TypeError: make_const_expr_f32(): incompatible function arguments. The following argument types are supported:
    1. (arg0: float) -> taichi_core.Expr

Invoked with: <Taichi 2x1 Matrix>

@pybind Hi, I know that printing Invoked with: xxx is nice to debug, but do you know that it's also possible for the repr of xxx to fail?
Can we catch it and say: During processing the above exception, another exception occurred instead of a corrupt stack backtrace?

[xumingkuan]

LGTM!

[rexwangcc]

LGTM!