Fix bare except clauses in abstract_solver.py, constraints.py, and monitors.py

[koteshyelamati]

Fix bare except: clauses that catch BaseException, replacing them with specific exception types.

mystic/abstract_solver.py (line 560):
except: → except (TypeError, ValueError): — wraps float(var) which can raise TypeError (non-numeric) or ValueError (bad literal).

mystic/constraints.py (line 349):
except: → except AttributeError: — wraps guess.copy() which raises AttributeError when the object has no .copy() method.

mystic/monitors.py (line 643):
except: → except Exception: — wraps exec(code, _globals) which can raise various exceptions at runtime.

Bare except: clauses are bad practice (PEP 8 / flake8 E722) because they silently swallow KeyboardInterrupt and SystemExit, making programs uninterruptible.