Throw exception on invalid expression instead of giving PHP error

[Kekos]

When math_eval() is given an invalid math expression, the MathParser package returns a null AST. This PR changes the behaviour by throwing an InvalidExpression exception instead which make it catchable.

[coveralls]

Pull Request Test Coverage Report for Build 23

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 21:	0.0%
Covered Lines:	13
Relevant Lines:	13

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 23

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 21:	0.0%
Covered Lines:	13
Relevant Lines:	13

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 23

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 21:	0.0%
Covered Lines:	13
Relevant Lines:	13

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 23

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 21:	0.0%
Covered Lines:	13
Relevant Lines:	13

---

💛 - Coveralls