Improve error messaging around Pauli string parsing

[karalekas]

Following on from #1007, @jlbosse also improved in #984 some of the error messages for PauliSum and PauliTerm objects, but they certainly could continue to improve. A valid approach would be to separate errors into those concerned with operators and those concerned with coefficients, and to create custom exceptions that point users in the right direction for creating a valid object from a Pauli string.

In [5]: PauliTerm.from_compact_str("(1.0+9i)*X0")
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/code/rigetti/github/pyquil/pyquil/paulis.py in from_compact_str(cls, str_pauli_term)
    371         try:
--> 372             coef = float(str_coef)
    373         except ValueError:

ValueError: could not convert string to float: '(1.0+9i)'

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
~/code/rigetti/github/pyquil/pyquil/paulis.py in from_compact_str(cls, str_pauli_term)
    374             try:
--> 375                 coef = complex(str_coef)
    376             except ValueError:

ValueError: complex() arg is a malformed string

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
<ipython-input-5-e7b8a524abb7> in <module>
----> 1 PauliTerm.from_compact_str("(1.0+9i)*X0")

~/code/rigetti/github/pyquil/pyquil/paulis.py in from_compact_str(cls, str_pauli_term)
    375                 coef = complex(str_coef)
    376             except ValueError:
--> 377                 raise ValueError("Could not parse the coefficient "
    378                                  f"{str_coef}")
    379

ValueError: Could not parse the coefficient (1.0+9i)

In [6]: PauliTerm.from_compact_str("(1.0+0j)*A0")
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-c2a2c5e2d3b7> in <module>
----> 1 PauliTerm.from_compact_str("(1.0+0j)*A0")

~/code/rigetti/github/pyquil/pyquil/paulis.py in from_compact_str(cls, str_pauli_term)
    385         str_op = re.sub(r'\*', '', str_op)
    386         if not re.match(r'^(([XYZ])(\d+))+$', str_op):
--> 387             raise ValueError(f"Could not parse operator string {str_op}. "
    388                              r"It should match ^(([XYZ])(\d+))+$")
    389

ValueError: Could not parse operator string A0. It should match ^(([XYZ])(\d+))+$