Throws SecurityTokenMalformedTokenException on malformed tokens

[dannybtsai]

Currently an ArgumentException is thrown when a token handler fails to parse/read a raw token. Since the ArgumentException is a general exception for any invalid arguments, sometimes it does not provide specific types of errors, ex, invalid header or payload. This PR adds a new exception, SecurityTokenMalformedTokenException, that can be thrown when a malformed token is detected. For example, the exception will be thrown if a SAML token is handled by JsonWebTokenHandler.

This should provide a clearer exception so users to handle it differently.

[victorm-hernandez]

    /// <exception cref="ArgumentException">'jwtEncodedString' contains only whitespace.</exception>

Let's update the XML docs to reflect we are throwing a different exception type.

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In reply to: 1551947809

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In reply to: 1551947809

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Refers to: src/System.IdentityModel.Tokens.Jwt/JwtSecurityToken.cs:24 in 1f748de. [](commit_id = 1f748de, deletion_comment = False)

[victorm-hernandez]

    /// <exception cref="ArgumentException"><paramref name="token"/>.Length is greater than <see cref="TokenHandler.MaximumTokenSizeInBytes"/>.</exception>

ditto: lets update XML docs for the new exception type

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In reply to: 1551949110

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In reply to: 1551949110

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Refers to: src/System.IdentityModel.Tokens.Jwt/JwtSecurityTokenHandler.cs:815 in 1f748de. [](commit_id = 1f748de, deletion_comment = False)

[victorm-hernandez]

    public override ClaimsPrincipal ValidateToken(string token, TokenValidationParameters validationParameters, out SecurityToken validatedToken)

Let's update also the JsonWebTokenHandler and Saml2SecurityTokenHandler ValidateToken methods to be consistent.

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In reply to: 1551955852

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Refers to: src/System.IdentityModel.Tokens.Jwt/JwtSecurityTokenHandler.cs:835 in 1f748de. [](commit_id = 1f748de, deletion_comment = False)

[victorm-hernandez]

🕐

[victorm-hernandez]

    public override ClaimsPrincipal ValidateToken(string token, TokenValidationParameters validationParameters, out SecurityToken validatedToken)

Danny created a work item for this
https://identitydivision.visualstudio.com/Engineering/_workitems/edit/2589877

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In reply to: 1551955852

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Refers to: src/System.IdentityModel.Tokens.Jwt/JwtSecurityTokenHandler.cs:835 in 1f748de. [](commit_id = 1f748de, deletion_comment = False)

[victorm-hernandez]